Semiconductor device

ABSTRACT

An object is to provide a memory device including a memory element that can be operated without problems by a thin film transistor with a low off-state current. Provided is a memory device in which a memory element including at least one thin film transistor that includes an oxide semiconductor layer is arranged as a matrix. The thin film transistor including an oxide semiconductor layer has a high field effect mobility and low off-state current, and thus can be operated favorably without problems. In addition, the power consumption can be reduced. Such a memory device is particularly effective in the case where the thin film transistor including an oxide semiconductor layer is provided in a pixel of a display device because the memory device and the pixel can be formed over one substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/671,199, filed Aug. 8, 2017, now allowed, which is a continuation ofU.S. application Ser. No. 15/296,432, filed Oct. 18, 2016, now U.S. Pat.No. 9,735,285, which is a continuation of U.S. application Ser. No.14/989,927, filed Jan. 7, 2016, now U.S. Pat. No. 9,478,564, which is acontinuation of U.S. application Ser. No. 14/336,142, filed Jul. 21,2014, now U.S. Pat. No. 9,236,385, which is a continuation of U.S.application Ser. No. 12/907,722, filed Oct. 19, 2010, now U.S. Pat. No.8,803,142, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2009-242871 on Oct. 21, 2009, all of whichare incorporated by reference.

TECHNICAL FIELD

The present invention relates to semiconductor devices. In particular,the present invention relates to semiconductor devices including memorycircuits (e.g., memory devices).

BACKGROUND ART

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. A transistor is used for a display device typifiedby a liquid crystal television. Silicon is known as a material for asemiconductor thin film applicable to a transistor; however, an oxidesemiconductor has attracted attention in recent years.

As a material for the oxide semiconductor, zinc oxide or a materialcontaining zinc oxide as its component is known (e.g., Patent Documents1 and 2). Further, a transistor formed using an amorphous oxide (oxidesemiconductor) having an electron carrier concentration of less than10¹⁸ cm⁻³ is disclosed (e.g., Patent Document 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2006-165529

DISCLOSURE OF INVENTION

However, a difference from the stoichiometric composition in the oxidesemiconductor arises in a thin film formation process. The electricconductivity of the oxide semiconductor changes due to the excess ordeficiency of oxygen. Further, hydrogen that enters the oxidesemiconductor during the formation of the thin film of the oxidesemiconductor bonds with oxygen (an O—H bond is formed) and serves as anelectron donor, which is a factor of changing the electric conductivity.Furthermore, since the O—H bond is a polar bond, it serves as a factorof varying characteristics of an active device such as a transistormanufactured using an oxide semiconductor.

Even when having an electron carrier concentration of less than 10¹⁸cm⁻³, an oxide semiconductor is a substantially n-type semiconductor,and an on-off ratio of the transistor disclosed in the Patent Documents1 to 3 is about 10³. Such a low on-off ratio of the transistor is due tolarge off-state current. Here, the on-off ratio refers to a ratio of thevalue of on-state current to the value of off-state current.

In view of the above problems, it is an object of one embodiment of thepresent invention to provide a transistor having stable electriccharacteristics (e.g., excessively-low off-state current).

One embodiment of the present invention is a semiconductor device whosememory circuit includes a transistor in which a channel formation regionis formed using an oxide semiconductor film. Hydrogen or OH groupcontained in the oxide semiconductor is removed so that theconcentration of hydrogen contained in the oxide semiconductor is lessthan or equal to 5×10¹⁹ cm⁻³, preferably less than or equal to 5×10¹⁸cm⁻³, more preferably less than or equal to 5×10¹⁷ cm⁻³, and carrierconcentration is less than or equal to 5×10¹⁴ cm⁻³, preferably less thanor equal to 5×10¹² cm⁻³.

Here, the energy gap of the oxide semiconductor film is set to 2 eV ormore, preferably 2.5 eV or more, still preferably 3 eV or more to reduceas many impurities (e.g., hydrogen), which form donors, as possible.Further, the carrier concentration of the oxide semiconductor film isset to less than or equal to 1×10¹⁴ cm⁻³, preferably less than or equalto 1×10¹² cm⁻³.

The thus highly-purified oxide semiconductor is used for a channelformation region of a transistor. Accordingly, even in the case wherethe channel width is extremely long, which is 10 mm, the drain currentof 1×10⁻¹³ A or less can be obtained at drain voltages of 1 V and 10 Vand gate voltages in the range of −5 V to −20 V.

With one embodiment of the present invention, by forming a circuit byusing a transistor that includes a highly-purified oxide semiconductorfilm, stable operation of the circuit can be obtained. Since off-statecurrent is lowered to less than or equal to 1×10⁻¹³ A, a display deviceincluding the above-described circuit does not need to be additionallyprovided with a capacitor for holding a signal voltage applied to apixel. That is, in the case where the circuit is applied to a displaydevice, a storage capacitor is unnecessary for each pixel; thus,aperture ratio can be improved.

With one embodiment of the present invention, a display device includingthe above-described circuit can hold a fixed state of a pixel (e.g., astate in which an image signal is written); thus, stable operation canbe obtained also in the case where a still image is displayed.

With one embodiment of the present invention, high field-effect mobilityand low off-state current can be obtained; thus, a semiconductor deviceincluding a memory circuit that is operated favorably without problemsand that has low power consumption can be obtained. A semiconductordevice including such a memory circuit is particularly effective in thecase where a transistor including an oxide semiconductor layer isprovided in a pixel of a display device because the semiconductor deviceincluding the memory circuit and the pixel can be manufactured over onesubstrate.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1E illustrate a transistor which is one embodiment of thepresent invention and a method for manufacturing the transistor, andFIG. 1F illustrates a capacitor which is one embodiment of the presentinvention.

FIG. 2 illustrates a transistor which is one embodiment of the presentinvention.

FIG. 3 shows initial properties of a transistor which is one embodimentof the present invention.

FIGS. 4A and 4B each show a transistor which is one embodiment of thepresent invention.

FIGS. 5A and 5B each show V_(g)-I_(d) characteristics of a transistorwhich is one embodiment of the present invention.

FIG. 6 illustrates a memory circuit (a DRAM) which is one embodiment ofthe present invention.

FIG. 7 illustrates a refresh circuit of a memory circuit which is oneembodiment of the present invention.

FIG. 8 illustrates a memory circuit (an SRAM) which is one embodiment ofthe present invention.

FIG. 9 illustrates a memory circuit (an SRAM) which is one embodiment ofthe present invention.

FIGS. 10A to 10D illustrate a transistor which is one embodiment of thepresent invention and a method for manufacturing the transistor.

FIGS. 11A to 11D illustrate a transistor which is one embodiment of thepresent invention and a method for manufacturing the transistor.

FIGS. 12A and 12B illustrate a transistor which is one embodiment of thepresent invention.

FIGS. 13A to 13E illustrate a transistor which is one embodiment of thepresent invention and a method for manufacturing the transistor.

FIGS. 14A and 14B each illustrate a transistor which is one embodimentof the present invention.

FIGS. 15A and 15B illustrate a transistor which is one embodiment of thepresent invention.

FIGS. 16A to 16E illustrate a transistor which is one embodiment of thepresent invention and a method for manufacturing the transistor.

FIG. 17 illustrates a transistor which is one embodiment of the presentinvention.

FIGS. 18A and 18B are energy band diagrams on A-A′ in FIG. 17.

FIGS. 19A and 19B are energy band diagrams on B-B′ in FIG. 17.

FIG. 20 is a diagram illustrating a relation between a vacuum level anda work function of a metal (ϕ_(M)), and a relation between a vacuumlevel and an electron affinity of an oxide semiconductor (χ).

FIG. 21 illustrates a CPU which is one embodiment of the presentinvention.

FIGS. 22A to 22C illustrate a wireless communication semiconductordevice which is one embodiment of the present invention.

FIGS. 23A and 23B each illustrate a wireless communication semiconductordevice which is one embodiment of the present invention.

FIG. 24 illustrates a wireless communication semiconductor device whichis one embodiment of the present invention.

FIG. 25 illustrates a wireless communication semiconductor device whichis one embodiment of the present invention.

FIGS. 26A to 26F each illustrate an application example of a wirelesscommunication semiconductor device which is one embodiment of thepresent invention.

FIGS. 27A to 27C each illustrate an example of a display device which isone embodiment of the present invention.

FIGS. 28A to 28C each illustrate an example of an electronic appliancewhich is one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention are described withreference to the drawings. However, the present invention is not limitedto the following description. It is easily understood by those skilledin the art that the mode and detail can be variously changed unlessdeparting from the scope and spirit of the present invention. Therefore,the present invention is not construed as being limited to the followingdescription of the embodiments and examples. Note that referencenumerals denoting the same portions are commonly used in differentdrawings.

Note that the size, the thickness of a layer, or a region in structuresillustrated in the drawings and the like in the embodiments areexaggerated for simplicity in some cases. Therefore, embodiments of thepresent invention are not limited to such scales.

Note that terms with ordinal numbers such as “first”, “second”, and“third” in this specification are used in order to identify components,and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention is described.

First, a transistor which can be used for the semiconductor device ofthis embodiment is described with reference to FIGS. 1A to 1E.

A transistor 110 illustrated in FIGS. 1A to 1E is an inverted staggeredtransistor having a bottom-gate structure.

Although the transistor 110 is a single-gate transistor, a multi-gatetransistor having a plurality of channel formation regions may also beused as necessary.

First, after a conductive film is formed over a substrate 100 having aninsulating surface, a first photolithography step is performed, so thata gate electrode layer 111 is formed. Alternatively, the gate electrodelayer 111 may be formed by processing the conductive film with use of aresist mask formed by an inkjet method. In the case of forming a resistmask by an inkjet method, the manufacturing cost can be reduced becausea photomask is not used.

Although there is no particular limitation on a substrate that can beused as the substrate 100 having an insulating surface, it is necessarythat the substrate 100 having an insulating surface have at least enoughheat resistance to heat treatment to be performed later. As thesubstrate 100 having an insulating surface, a glass substrate such as abarium borosilicate glass substrate or an aluminoborosilicate glasssubstrate can be used.

As the glass substrate, a glass substrate formed using a material suchas aluminosilicate glass, aluminoborosilicate glass, or bariumborosilicate glass may be used. In the case where the temperature atwhich the heat treatment is performed later is high, it is preferable touse a glass substrate whose strain point is higher than or equal to 730°C. Note that more practical glass with heat resistance can be obtainedwhen it contains a larger amount of barium oxide (BaO) than diborontrioxide (B₂O₃). Therefore, a glass substrate containing BaO and B₂O₃ sothat the amount of BaO is larger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be usedinstead of the glass substrate. A crystallized glass substrate or thelike may also be used.

An insulating film serving as a base film may be provided between thesubstrate 100 and the gate electrode layer 111. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 100, and can be formed to have a single-layer structure orstacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

The gate electrode layer 111 can be formed to have a single-layerstructure or a stacked-layer structure using one or more of metalmaterials such as Mo, Ti, Cr, Ta, W, Al, Cu, Nd and Sc, and alloymaterials containing the above metal material as its main component. Inthe case where the gate electrode layer 111 has a two-layer structure, apreferable structure is, for example, a structure in which a Mo layer isstacked over an Al layer, a structure in which a Mo layer is stackedover a Cu layer, a structure in which a titanium nitride layer or atantalum nitride layer is stacked over a Cu layer, a structure in whicha titanium nitride layer and a Mo layer are stacked, or a structure inwhich a tungsten nitride layer and a W layer are stacked. In the casewhere the gate electrode layer 111 has a three-layer structure, apreferable structure is a structure in which a W layer or a tungstennitride layer, an Al—Si alloy layer or an Al—Ti alloy layer, and atitanium nitride layer or a Ti layer are stacked.

Then, a gate insulating layer 102 is formed over the gate electrodelayer 111.

The gate insulating layer 102 can be formed to have a single-layerstructure or a stacked-layer structure using one or more of a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, and an aluminum oxide layer by a plasma CVDmethod, a sputtering method, or the like. For example, a siliconoxynitride layer may be formed by a plasma CVD method using a SiH₄ gas,an oxygen gas, and a nitrogen gas as a source gas. The thickness of thegate insulating layer 102 is preferably more than or equal to 100 nm andless than or equal to 500 nm. In the case where the gate insulatinglayer 102 has a stacked-layer structure of a plurality of insulatinglayers, a stack of a first gate insulating layer having a thickness ofmore than or equal to 50 nm and less than or equal to 200 nm and asecond gate insulating layer having a thickness of more than or equal to5 nm and less than or equal to 300 nm over the first gate insulatinglayer may be used. The total thickness of the gate insulating layer 102,even in the case of a stacked-layer structure of a plurality ofinsulating layers, is preferably more than or equal to 100 nm and lessthan or equal to 500 nm.

In this embodiment, a silicon oxynitride layer having a thickness ofless than or equal to 100 nm is formed by a plasma CVD method as thegate insulating layer 102.

Next, an oxide semiconductor film 130 having a thickness of more than orequal to 2 nm and less than or equal to 200 nm is formed over the gateinsulating layer 102.

Note that before the oxide semiconductor film 130 is formed by asputtering method, dust attached to a surface of the gate insulatinglayer 102 is preferably removed by reverse sputtering in which plasma isgenerated in an argon gas atmosphere. Note that nitrogen, helium,oxygen, or the like may be used instead of the argon atmosphere.

The oxide semiconductor film 130 is formed using: a four-component metaloxide such as an In—Sn—Ga—Zn—O-based metal oxide; a three-componentmetal oxide such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-basedmetal oxide, an In—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metaloxide, an Al—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metaloxide; a two-component metal oxide such as an In—Zn—O-based metal oxide,a Sn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, or anIn—Sn—O-based metal oxide; an In—O-based metal oxide, a Sn—O-based metaloxide, or a Zn—O-based metal oxide. In this embodiment, the oxidesemiconductor film 130 is formed by a sputtering method with use of anIn—Ga—Zn—O-based oxide semiconductor target. FIG. 1A is across-sectional view at this stage. The oxide semiconductor film 130 canbe formed by a sputtering method in a rare gas (e.g., argon) atmosphere,an oxygen gas atmosphere, or a mixed gas atmosphere including a rare gas(e.g., argon) and oxygen. In the case of using a sputtering method, atarget including SiO₂ at more than or equal to 2 wt % and less than orequal to 10 wt % may be used.

As a target for forming the oxide semiconductor film 130 by a sputteringmethod, for example, a metal oxide target containing zinc oxide as itsmain component can be used. As another example of a metal oxide target,an oxide semiconductor target containing In, Ga, and Zn (in acomposition ratio, In₂O₃:Ga₂O₃:ZnO=1:1:1 [mol ratio], orIn:Ga:Zn=1:1:0.5 [atomic ratio]) can be used. As an oxide semiconductortarget containing In, Ga, and Zn (in a composition ratio, In:Ga:Zn=1:1:1[atomic ratio], or In:Ga:Zn=1:1:2 [atomic ratio]) can also be used. Thefill rate of the oxide semiconductor target is more than or equal to 90%and less than or equal to 100%, preferably, more than or equal to 95%and less than or equal to 99.9%. With the use of the oxide semiconductortarget with high fill rate, a dense oxide semiconductor film is formed.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the oxide semiconductor film 130 is formed. Note that thesputtering gas means a gas which is introduced into a treatment chamberwhere sputtering is performed.

First, the substrate is held inside a treatment chamber which is kept ina reduced pressure state, and the substrate is heated to a temperatureof more than or equal to 100° C. and less than or equal to 600° C.(preferably, more than or equal to 200° C. and less than or equal to400° C.). By forming the oxide semiconductor film in the state where thesubstrate is heated, the concentration of impurities in the oxidesemiconductor film formed and damage caused by sputtering can bereduced. Then, a sputtering gas from which hydrogen and moisture areremoved is introduced into the treatment chamber, and an oxidesemiconductor film is formed over the substrate with use of a metaloxide as a target in the state where moisture remaining in the treatmentchamber is removed. An entrapment vacuum pump is preferably used forevacuating the treatment chamber. For example, a cryopump, an ion pump,or a titanium sublimation pump is preferably used. The evacuation unitmay be a turbo pump provided with a cold trap. For example, a hydrogenatom, a compound containing a hydrogen atom such as water, a compoundcontaining a carbon atom, and the like are removed from the treatmentchamber which is evacuated with the cryopump; as a result, theconcentration of impurities in the oxide semiconductor film formed inthe treatment chamber can be reduced.

The oxide semiconductor film is formed, for example, under conditionswhere the distance between the substrate and the target is 100 mm, thepressure in the treatment chamber is 0.6 Pa, the direct current (DC)power supply is 0.5 kW, and the atmosphere is oxygen (the proportion ofthe oxygen flow is 100%). Note that a pulsed direct current (DC) powersupply is preferably used because generation of powder substances (alsoreferred to as particles or dust) in film formation can be suppressedand the film thickness can be uniform. Although the thickness of theoxide semiconductor film is preferably more than or equal to 5 nm andless than or equal to 30 nm, the thickness may be determined asappropriate depending on the oxide semiconductor material withoutlimitation.

Then, a second photolithography step is performed, so that the oxidesemiconductor film 130 is processed into an island-shaped oxidesemiconductor layer. Alternatively, the island-shaped oxidesemiconductor layer may be formed by processing the oxide semiconductorfilm 130 with use of a resist mask formed by an inkjet method. In thecase of forming a resist mask by an inkjet method, the manufacturingcost can be reduced because a photomask is not used.

Next, the oxide semiconductor layer is subjected to first heattreatment. With the first heat treatment, dehydration or dehydrogenationof the oxide semiconductor layer can be conducted. The temperature ofthe first heat treatment is more than or equal to 400° C. and less thanor equal to 750° C., preferably more than or equal to 400° C. and lessthan the strain point of the substrate. Here, the substrate 100 istransferred into an electric furnace which is one of heat treatmentapparatuses, heat treatment is performed on the oxide semiconductorlayer in a nitrogen atmosphere at 450° C. for one hour, and then, theoxide semiconductor layer is not exposed to the air so that entry ofwater and hydrogen into the oxide semiconductor layer is prevented;thus, an oxide semiconductor layer 131 is obtained (FIG. 1B).

The heat treatment apparatus is not limited to the above-describedelectric furnace but may be a heat treatment apparatus provided with aunit that heats an object to be processed by thermal conduction orthermal radiation from a heater such as a resistance heater or the like.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. Note that the GRTA apparatus is a heat treatmentapparatus using a high-temperature gas. As the high-temperature gas, aninert gas which is unlikely to react with an object to be processed byheat treatment, such as nitrogen gas or a rare gas like argon, is used.The LRTA apparatus is a heat treatment apparatus for heating the objectto be processed by radiation of light (an electromagnetic wave) emittedfrom a lamp such as a halogen lamp, a metal halide lamp, a xenon arclamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressuremercury lamp.

For example, as the first heat treatment, GRTA may be performed asfollows. The substrate is transferred into a treatment chamber filledwith a high-temperature gas at a temperature of 650° C. to 700° C.,subjected to the high-temperature gas for several minutes, and taken outfrom the treatment chamber. Such GRTA enables heat treatment for a shorttime.

Note that it is preferable that in the first heat treatment, nitrogen ora rare gas such as helium, neon, or argon, which is a gas introducedinto the treatment chamber does not contain water, hydrogen, and thelike. Alternatively, the purity of nitrogen or a rare gas such ashelium, neon, or argon which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or higher, more preferably 7N(99.99999%) or higher (that is, the impurity concentration is less thanor equal to 1 ppm, preferably less than or equal to 0.1 ppm).

There is a case where the oxide semiconductor layer is crystallized tobe a microcrystalline film or a polycrystalline film depending onconditions of the first heat treatment or materials of the oxidesemiconductor layer. For example, the oxide semiconductor layer may becrystallized into a microcrystalline oxide semiconductor film having adegree of crystallization of more than or equal to 90%, or more than orequal to 80%. There is also a case where the oxide semiconductor layerbecomes an amorphous oxide semiconductor film which does not containcrystalline components depending on conditions of the first heattreatment or materials of the oxide semiconductor layer. Alternatively,there is a case where an amorphous film contains mirocrystals (the grainsize is more than or equal to 1 nm and less than or equal to 20 nm(e.g., more than or equal to 2 nm and less than or equal to 4 nm)).

The first heat treatment of the oxide semiconductor layer may beperformed on the oxide semiconductor film 130 before being processedinto the island-shaped oxide semiconductor layer. In that case, afterthe first heat treatment, the substrate is taken out of the heattreatment apparatus, and then a photolithography step is performed, sothat the island-shaped oxide semiconductor layer is formed.

The heat treatment having effects of dehydration or dehydrogenation ofthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode and a drain electrode are formed over the oxide semiconductorlayer; and after a protective insulating film is formed over the sourceelectrode and the drain electrode.

In the case where a contact hole is formed in the gate insulating layer102, the contact hole in the gate insulating layer 102 may be formedbefore dehydration or dehydrogenation treatment is performed on theoxide semiconductor film 130, or after the dehydration ordehydrogenation treatment is performed.

Next, a conductive film is formed over the gate insulating layer 102 andthe oxide semiconductor layer 131. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As a material of theconductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W;an alloy containing any of these elements as a component; an alloy filmcontaining any of these elements in combination; and the like can begiven. In addition, one or more of Mn, Mg, Zr, Be, and Th may also beused. The conductive film may have a single-layer structure or astacked-layer structure of a plurality of layers. For example, asingle-layer structure of an Al film containing Si, a two-layerstructure in which a Ti film is stacked over an Al film, a three-layerstructure in which an Al film is interposed between two Ti films, or thelike can be given. When the conductive film includes an Al film, it maycontain Nd or Sc. Note that these films may be nitride films.

Next, a third photolithography step is performed, so that the conductivefilm is processed into a source electrode layer 115 a and a drainelectrode layer 115 b. After that, a resist mask is removed (FIG. 1C).

KrF laser light or ArF laser light is preferably used for light exposurefor forming the resist mask in the third photolithography step. Channellength L of a transistor to be formed later is determined by a spacebetween the source electrode layer and the drain electrode layer, whichare adjacent to each other over the oxide semiconductor layer 131. Notethat in the case where light exposure is performed so that the channellength L becomes less than 25 nm, light exposure for forming the resistmask in the third photolithography step is performed using extremeultraviolet with an extremely short wavelength of several nanometers toseveral tens of nanometers. Light exposure using extreme ultravioletenables high resolution and deep depth of focus. Thus, the channellength L of the transistor to be formed later can be more than or equalto 10 nm and less than or equal to 1000 nm and the operation speed of acircuit can be increased, and furthermore the value of off-state currentis extremely small, and thus lower power consumption can be achieved.

Note that each material and etching conditions need to be adjusted asappropriate so that the oxide semiconductor layer 131 is not removed inetching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, and anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 131. As an etchant, an ammonia hydrogen peroxide mixture (a mixedsolution of ammonia, water, and a hydrogen peroxide solution) is used.

In the third photolithography step, only part of the oxide semiconductorlayer 131 is etched off, and an oxide semiconductor layer having agroove (a depressed portion) is formed in some cases. A resist mask forforming the source electrode layer 115 a and the drain electrode layer115 b may be formed by an inkjet method. In the case of forming a resistmask by an inkjet method, the manufacturing cost can be reduced becausea photomask is not used.

Further, an oxide conductive layer may be formed between the oxidesemiconductor layer 131 and the source electrode layer 115 a and betweenthe oxide semiconductor layer 131 and the drain electrode layer 115 b.The oxide conductive layer and the conductive layer for forming thesource and drain electrode layers can be successively formed. The oxideconductive layer can function as source and drain regions.

By forming the oxide conductive layer between the oxide semiconductorlayer and the source and drain electrode layers as the source and drainregions, the source and drain regions can have lower resistance and thetransistor can be operated at high speed.

In order to reduce the number of photomasks and steps in thephotolithography step, the etching step may be performed using a resistmask formed by a multi-tone mask.

Next, plasma treatment is performed using a dinitrogen monoxide gas, anitrogen gas, an Ar gas, or the like. Through this plasma treatment,water or the like adsorbed on a surface of the oxide semiconductor layerwhich is exposed is removed. Alternatively, plasma treatment may beperformed using a mixed gas of an oxygen gas and Ar. After the plasmatreatment, the insulating oxide layer 116 which serves as a protectiveinsulating film and is in contact with a surface of the exposed oxidesemiconductor layer is formed without exposure to the air.

The insulating oxide layer 116 can be formed to a thickness of more thanor equal to 1 nm by a method with which impurities such as water orhydrogen does not enter the insulating oxide layer 116 (e.g., asputtering method or the like). When hydrogen is contained in theinsulating oxide layer 116, entry of the hydrogen to the oxidesemiconductor layer or extraction of oxygen in the oxide semiconductorlayer by the hydrogen is caused, whereby part of the oxide semiconductorlayer that is in contact with the insulating oxide layer 116 comes to ben-type (to have a lower resistance) and thus a parasitic channel mightbe formed. Therefore, it is important that the insulating oxide layer116 is formed containing as little hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is formed by asputtering method as the insulating oxide layer 116. The substratetemperature in film formation may be more than or equal to roomtemperature and less than or equal to 300° C., and is 100° C. in thisembodiment. The formation of the silicon oxide film by a sputteringmethod can be performed in a rare gas (e.g., argon) atmosphere, anoxygen gas atmosphere, or an atmosphere of a mixed gas of a rare gas(e.g., argon) and an oxygen gas. Further, a silicon oxide target or asilicon target can be used as a target. For example, with the use of asilicon target, a silicon oxide film can be formed by a sputteringmethod in a mixed gas atmosphere of an oxygen gas and a nitrogen gas. Asthe insulating oxide layer 116 which is formed in contact with the oxidesemiconductor layer whose resistance is reduced, an inorganic insulatingfilm which does not contain impurities such as moisture, hydrogen ions,and hydroxy groups and blocks entry of these from the outside is used;for example, a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, an aluminum oxynitride film, or the like is used.

In that case, the insulating oxide layer 116 is preferably formed in thestate where moisture remaining in the treatment chamber is removed. Thisis for preventing hydrogen, hydroxy groups, or moisture from beingcontained in the oxide semiconductor layer 131 and the insulating oxidelayer 116.

As described above, an entrapment vacuum pump is preferably used forevacuating the treatment chamber. The concentration of impuritiescontained in the insulating oxide layer 116 can be reduced by using theentrapment vacuum pump.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the insulating oxide layer 116 is formed.

Next, second heat treatment is performed. The temperature of the secondheat treatment is preferably more than or equal to 200° C. and less thanor equal to 400° C., and more preferably more than or equal to 250° C.and less than or equal to 350° C. Note that the second heat treatmentmay be performed in an inert gas atmosphere or an oxygen gas atmosphere.In this embodiment, heat treatment may be performed in a nitrogenatmosphere at 250° C. for one hour. The second heat treatment isperformed in the state where part of the oxide semiconductor layer (achannel formation region) is in contact with the insulating oxide layer116.

Through the above process, heat treatment for dehydration ordehydrogenation is performed on the deposited oxide semiconductor filmto reduce the resistance, and then, part of the oxide semiconductor filmis selectively made in an oxygen-excess state. As a result, the channelformation region 113 overlapping with the gate electrode layer 111becomes an i-type semiconductor having a low conductivity, and ahigh-resistance source region 114 a overlapping with the sourceelectrode layer 115 a and a high-resistance drain region 114 boverlapping with the drain electrode layer 115 b are formed in aself-aligned manner. Through the above process, the transistor 110 isformed (FIG. 1D).

Here, heat treatment may further be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. In this embodiment,the heat treatment is performed at 150° C. for 10 hours. This heattreatment may be performed at a fixed heating temperature, or may beperformed by repeating increasing a temperature from room temperature toa heat temperature of more than or equal to 100° C. and less than orequal to 200° C. and decreasing the temperature from the heattemperature to room temperature plural times. Further, this heattreatment may be performed before formation of the insulating oxidelayer under a reduced pressure. When the heat treatment is performedunder a reduced pressure, the heating time can be shortened. With thisheat treatment, a normally-off transistor can be obtained. In the caseof using a silicon oxide layer which contains a large number of defectsas the insulating oxide layer, the defects can be reduced by this heattreatment.

By the formation of the high-resistance drain region 114 b (or thehigh-resistance source region 114 a) in part of the oxide semiconductorlayer, which overlaps with the drain electrode layer 115 b (and thesource electrode layer 115 a), the reliability of the transistor can beimproved. Specifically, by forming the high-resistant drain region 114b, it is possible to employ a structure in which conductivity can begradually varied from the drain electrode layer 115 b to thehigh-resistant drain region 114 b and the channel formation region 113.Therefore, in the case where the transistor is operated in the statewhere the drain electrode layer 115 b is connected to a wiring forsupplying a high power supply potential Vdd, even when a differencebetween potentials of the gate electrode layer 111 and the drainelectrode layer 115 b is large, the high-resistance drain region 114 bserves as a buffer, so that the withstand voltage of the transistor canbe increased.

In the case where the thickness of the oxide semiconductor layer is lessthan or equal to 15 nm, the high-resistance source region 114 a and thehigh-resistance drain region 114 b are formed entirely in the depthdirection; whereas in the case where the thickness of the oxidesemiconductor layer more than or equal to 30 nm and less than or equalto 50 nm, the high-resistance source region 114 a and thehigh-resistance drain region 114 b are formed in part of the oxidesemiconductor layer (a region in contact with the source and drainelectrode layers and the vicinity thereof), and a region of the oxidesemiconductor layer, which is near the gate insulating film, can becomean i-type semiconductor.

A protective insulating layer may be additionally formed over theinsulating oxide layer 116. For example, a silicon nitride film isformed as the protective insulating layer by an RF sputtering method.The RF sputtering method is preferable as a formation method of theprotective insulating layer because it achieves high mass productivity.The protective insulating layer is formed using an inorganic insulatingfilm which does not contain impurities such as moisture, hydrogen ions,and hydroxy groups and blocks entry of these from the outside; forexample, a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, an aluminum nitride oxide film, or the like is used.Here, a silicon nitride film is used to form a protective insulatinglayer 103 (FIG. 1E).

In this embodiment, the substrate 100 over which layers up to andincluding the insulating oxide layer 116 are formed is transferred intoa treatment chamber and heated at a temperature of more than or equal to100° C. and less than or equal to 400° C., a sputtering gas containing ahigh-purity nitrogen gas from which hydrogen and moisture are removed isintroduced into the treatment chamber, and a silicon nitride film isformed as the protective insulating layer 103 with use of a silicontarget. The protective insulating layer 103 is preferably formed in thestate where moisture remaining in the treatment chamber is removed, asin formation of the insulating oxide layer 116.

Note that a planarization insulating layer for planarization may beprovided over the protective insulating layer 103.

In the manner described above with reference to FIGS. 1A to 1E, thesemiconductor device of this embodiment can be manufactured. Note that acapacitor can also be formed in the above process. The capacitor isformed with the gate electrode layer 111, an electrode layer that isformed using the same material as the source electrode layer 115 a (andthe drain electrode layer 115 b), and the gate insulating layer 102. Thegate insulating layer 102 is provided between the gate electrode layer111 and the electrode layer (see FIG. 1F). Note also that in the casewhere another transistor in this specification is formed, the capacitormay be formed with a gate electrode layer, an electrode layer that isformed using the same material as a source electrode layer, and a gateinsulating layer between the gate electrode layer and the electrodelayer that is formed using the same material as the source electrodelayer. Note that the semiconductor device of this embodiment is notlimited to this mode, and a mode illustrated in FIG. 2 may also beemployed. FIG. 2 illustrates a transistor 140.

In manufacture of the transistor 140 illustrated in FIG. 2, first, agate electrode layer 121 is formed over a substrate 120, a first gateinsulating layer 122 a is formed, and the second gate insulating layer122 b is formed so as to be stacked thereover. In this embodiment, thegate insulating layer has a two-layer stacked structure; the first gateinsulating layer 122 a is formed using an insulating nitride layer, andthe second gate insulating layer 122 b is formed using an insulatingoxide layer.

The insulating oxide layer can be formed using a silicon oxide layer, asilicon oxynitride layer, an aluminum oxide layer, an aluminumoxynitride layer, or the like. The insulating nitride layer can beformed using a silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, an aluminum nitride oxide layer, or the like.

In this embodiment, a structure in which a silicon nitride layer and asilicon oxide layer are stacked in this order over the gate electrodelayer 121 is employed. The gate insulating layer (in this embodiment,the thickness is 150 nm) is formed by forming a silicon nitride layer(SiNy (y>0)) as the first gate insulating layer 122 a by a sputteringmethod to a thickness of more than or equal to 50 nm and less than orequal to 200 nm (in this embodiment, the thickness is 50 nm), and asilicon oxide layer (SiOx (x>0)) as the second gate insulating layer 122b by a sputtering method to a thickness of more than or equal to 5 nmand less than or equal to 300 nm (in this embodiment, the thickness is100 nm) over the first gate insulating layer 122 a.

Next, an oxide semiconductor film is formed over the gate insulatinglayer, and a photolithography step is performed, so that the oxidesemiconductor film is processed into an island-shaped oxidesemiconductor layer. In this embodiment, the oxide semiconductor film isformed by a sputtering method with use of an In—Ga—Zn—O-based oxidesemiconductor target.

The oxide semiconductor film is preferably formed in the state wheremoisture remaining in the treatment chamber is removed. This is forpreventing hydrogen, hydroxy groups, or moisture from being contained inthe oxide semiconductor film Note that an entrapment vacuum pump ispreferably used for evacuating the treatment chamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the oxide semiconductor film is formed.

Next, first heat treatment is performed, so that dehydration ordehydrogenation of the oxide semiconductor layer is conducted. Thetemperature of the first heat treatment is more than or equal to 400° C.and less than or equal to 750° C., preferably more than or equal to 400°C. and less than the strain point of the substrate. Note that in thecase where the temperature is more than or equal to 425° C., the heattreatment time may be less than or equal to one hour, whereas in thecase where the temperature is less than 425° C., the heat treatment timeis preferably more than one hour. Here, the substrate is transferredinto an electric furnace which is one of heat treatment apparatuses,heat treatment is performed on the oxide semiconductor layer in anitrogen atmosphere, and then, the oxide semiconductor layer is notexposed to the air so that entry of water and hydrogen into the oxidesemiconductor layer is prevented; thus, an oxide semiconductor layer isobtained. After that, a high-purity oxygen gas, a high-purity N₂O gas,or an ultra-dry air (having a dew point of lower than or equal to −40°C., preferably lower than or equal to −60° C.) is introduced into thesame furnace, and cooling is performed. It is preferable that the oxygengas or the N₂O gas do not contain water, hydrogen, and the like.Alternatively, the purity of an oxygen gas or an N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N (99.9999%)or higher, more preferably 7N (99.99999%) or higher (that is, theimpurity concentration of the oxygen gas or the N₂O gas is less than orequal to 1 ppm, preferably less than or equal to 0.1 ppm).

The heat treatment apparatus used here is not limited to the electricfurnace but may be a heat treatment apparatus provided with a unit thatheats an object to be processed by thermal conduction or thermalradiation from a heater such as a resistance heater or the like. Forexample, an RTA apparatus such as a GRTA apparatus or an LRTA apparatusmay be used. In this embodiment, the heat treatment is performed at 600°C. to 750° C. for several minutes using an RTA method.

Further, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at more than or equalto 200° C. and less than or equal to 400° C., preferably more than orequal to 200° C. and less than or equal to 300° C., in an atmosphere ofan oxygen gas or an N₂O gas.

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer. In that case, after thefirst heat treatment, the substrate is taken out of the heat treatmentapparatus, and then a photolithography step is performed.

Through the above process, the entire region of the oxide semiconductorfilm is made in an oxygen-excess state, thereby having higher resistance(becoming an i-type semiconductor); accordingly, an oxide semiconductorlayer 132 whose entire region is an i-type semiconductor is obtained.

Next, a conductive film is formed over the second gate insulating layer122 b and the oxide semiconductor layer 132. A photolithography step isperformed and a resist mask is formed over the conductive film, andselective etching is performed, so that the source electrode layer 135 aand the drain electrode layer 135 b are formed. Then, the resist mask isremoved. After that, an insulating oxide layer 136 is formed by asputtering method.

Note that the insulating oxide layer 136 is preferably formed in thestate where moisture remaining in the treatment chamber is removed. Thisis for preventing hydrogen, hydroxy groups, or moisture from beingcontained in the oxide semiconductor layer 132 and the insulating oxidelayer 136. An entrapment vacuum pump is preferably used for evacuatingthe treatment chamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the insulating oxide layer 136 is formed.

Next, in order to reduce variation in electric characteristics of thetransistor on the substrate, heat treatment (preferably at more than orequal to 150° C. and less than 350° C.) may be performed in an inert gasatmosphere or a nitrogen gas atmosphere. In this embodiment, the heattreatment is performed in a nitrogen atmosphere at 250° C. for one hour.

In addition, heat treatment may be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. In this embodiment,the heat treatment is performed at 150° C. for 10 hours. This heattreatment may be performed at a fixed heating temperature, or may beperformed by repeating increasing a temperature from room temperature toa heat temperature of more than or equal to 100° C. and less than orequal to 200° C. and decreasing the temperature from the heattemperature to room temperature plural times. Further, this heattreatment may be performed before formation of the insulating oxidelayer under a reduced pressure. When the heat treatment is performedunder a reduced pressure, the heating time can be shortened. With thisheat treatment, a normally-off transistor can be obtained.

Next, a protective insulating layer 123 is formed over the insulatingoxide layer 136. Here, a 100-nm-thick silicon nitride film is formed asthe protective insulating layer 123 by a sputtering method.

By forming the protective insulating layer 123 and the first gateinsulating layer 122 a using insulating nitride layers, it is possiblethat these layers do not contain impurities such as moisture, hydrogen,hydrides, and hydroxides, and block entry of these from the outside.

Thus, in this manufacturing process, entry of moisture and the like fromthe outside can be prevented. Further, even after a device is completedas a semiconductor device such as a liquid crystal display device, entryof impurities such as moisture from the outside can be prevented in thelong term; therefore, the long-term reliability of the device can beimproved.

Further, part of the insulating layers between the protective insulatinglayer 123 formed using an insulating nitride layer and the first gateinsulating layer 122 a may be removed so that the protective insulatinglayer 123 and the first gate insulating layer 122 a are in contact witheach other.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 123.

In the manner described above with reference to FIG. 2, thesemiconductor device of this embodiment can be manufactured.

Note that the temperature of the heat treatment in this embodiment isnot limited to the temperatures described above. As described below, thetemperature of the heat treatment may be less than 400° C.

A substrate over which a gate electrode layer is formed or a substrateover which the layers up to and including the gate insulating layer areformed is preferably preheated in a preheat chamber of a sputteringapparatus as pretreatment, and impurities such as hydrogen or moistureadsorbed on the substrate are preferably eliminated and evacuated, inorder that the gate insulating layer and the oxide semiconductor filmcontain as little hydrogen, hydroxy groups, and moisture as possible.The temperature of the preheating is more than or equal to 100° C. andless than 400° C. (preferably more than or equal to 150° C. and lessthan or equal to 300° C.). The preheat chamber is preferably providedwith a cryopump and evacuated with the cryopump. Note that thispretreatment of preheating may be performed on the substrate over whichthe layers up to and including the source and drain electrode layers areformed before formation of the insulating oxide layer, in a similarmanner.

The above pretreatment can be performed in manufacture of a liquidcrystal display panel, an electroluminescence display panel, and abackplane (a substrate over which a transistor is formed) of a displaydevice using electronic ink. The above pretreatment is performed at atemperature of less than 400° C.; therefore, it can be applied to amanufacturing process in which a glass substrate having a thickness ofless than or equal to 1 mm and having a side that is longer than 1 m isused. All the above steps can be performed at a temperature of less than400° C.; thus, a large amount of energy is not needed for manufacturinga display panel.

Although a sputtering method is used for formation of the gateinsulating layer in manufacture of the above-described transistor, themethod for formation of the gate insulating layer is not limited to themethod described above. For example, a high-density plasma CVD methodusing a microwave (2.45 GHz) may also be used.

Next, measured values of off-state current using a test element group(also referred to as a TEG) over which the above-described transistor isformed are described below.

FIG. 4A shows a top view in which 200 transistors each with L/W=3 μm/50μm (which are virtually identical to a transistor with L/W=3 μm/10000μm) are connected in parallel. FIG. 4B shows a partially enlarged topview thereof. The region enclosed by a dotted line in FIG. 4B is atransistor with L/W=3 μm/50 μm and Lov=1.5 μm. In measurement, thesubstrate temperature was set to room temperature. FIG. 3 showsV_(g)-I_(d) curves as transfer characteristics of the source-draincurrent (a drain current I_(d)) when the voltage between source anddrain (a drain voltage V_(d)) was set to 1 V or 10 V, and the voltagebetween source and gate (a gate voltage V_(g)) was varied from −20 V to+20 V. Note that FIG. 3 shows V_(g) in the range of from −20 V to +5 V.

As shown in FIG. 3, the off-state current is less than or equal to1×10⁻¹³ [A] at V_(d) of 1 V and 10 V. This value is less than or equalto the resolution (100 fA) of a measurement device (a semiconductorparameter analyzer, Agilent 4156C manufactured by Agilent TechnologiesInc.). A method for manufacturing such a film transistor with anextremely low off-state current is described below.

First, a silicon nitride layer was formed as a base layer over a glasssubstrate by a CVD method, and a silicon oxynitride layer was formedover the silicon nitride layer. A tungsten film was formed as a gateelectrode layer over the silicon oxynitride layer by a sputteringmethod. Here, the gate electrode layer was formed by selectively etchingthe tungsten film.

Then, a silicon oxynitride layer having a thickness of 100 nm was formedas a gate insulating layer over the gate electrode layer by a CVDmethod.

Then, an oxide semiconductor film having a thickness of 50 nm was formedover the gate insulating layer by a sputtering method using anIn—Ga—Zn—O-based oxide semiconductor target (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molratio]). Here, an island-shaped oxide semiconductor layer was formed byselectively etching the oxide semiconductor film.

Then, heat treatment was performed on the oxide semiconductor layer in anitrogen atmosphere with use of a clean oven at 450° C. for 1 hour(first heat treatment).

Then, a titanium layer (having a thickness of 150 nm) was formed as asource electrode layer and a drain electrode layer over the oxidesemiconductor layer by a sputtering method. Here, the source electrodelayer and the drain electrode layer were formed by selective etchingsuch that 200 transistors each having a channel length L of 3 μm and achannel width W of 50 μm were connected in parallel to obtain atransistor virtually with L/W=3 μm/10000 μm.

Then, a silicon oxide layer having a thickness of 300 nm was formed as aprotective insulating layer in contact with the oxide semiconductorlayer by a reactive sputtering method. Then, openings were formed overthe gate electrode layer, the source electrode layer, and the drainelectrode layer by selectively etching the silicon oxide layer. Afterthat, heat treatment was performed in a nitrogen atmosphere at 250° C.for 1 hour (second heat treatment). Then, heat treatment was performedin an inert gas atmosphere at 150° C. for 10 hours before themeasurement of V_(g)-I_(d) characteristics. Through the above process,an inverted staggered transistor having a bottom-gate structure wasmanufactured.

The reason why the transistor has an extremely small off-state current,which is less than or equal to 1×10⁻¹³ [A] as shown in FIG. 3, is thatthe concentration of hydrogen in the oxide semiconductor layer was ableto be sufficiently reduced in the above manufacturing process. Theconcentration of hydrogen in the oxide semiconductor layer is less thanor equal to 5×10¹⁹ cm⁻³, preferably less than or equal to 5×10¹⁸ cm⁻³,still preferably less than or equal to 5×10¹⁷ cm⁻³. Note that theconcentration of hydrogen in the oxide semiconductor layer may bemeasured by secondary ion mass spectroscopy (SIMS).

Although the example of using an In—Ga—Zn—O-based oxide semiconductor isdescribed here, this embodiment is not particularly limited thereto.Another oxide semiconductor material such as an In—Sn—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxidesemiconductor, an In—Zn—O-based oxide semiconductor, an In—Sn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, an In—O-based oxide semiconductor, aSn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor, canalso be used. Furthermore, as an oxide semiconductor material, anIn—Al—Zn—O-based oxide semiconductor mixed with AlO_(x) of 2.5 wt % to10 wt % or an In—Zn—O-based oxide semiconductor mixed with SiO_(x) of2.5 wt % to 10 wt % can be used.

The carrier concentration of the oxide semiconductor layer is less thanor equal to 5×10¹⁴ cm⁻³, preferably less than or equal to 5×10¹² cm⁻³,more preferably less than or equal to 1.45×10¹⁰ cm⁻³. That is, thecarrier concentration of the oxide semiconductor layer can be made asclose to zero as possible.

The channel length L of a transistor can be more than or equal to 10 nmand less than or equal to 1000 nm.

In circuit design, the oxide semiconductor layer can be regarded as aninsulator when the transistor is in an off state.

After that, the temperature dependence of off-state current of thetransistor was evaluated, and the results are shown. Temperaturedependence is important in considering the environmental resistance,maintenance of performance, or the like of an end product in which thetransistor is used. It is to be understood that smaller temperaturedependence of off-state current is more preferable, which increases thedegree of freedom for product designing.

Substrates over which transistors were formed were kept at a certaintemperature by using a constant-temperature bath, the drain voltage was6V, and the gate voltage was varied from −20 V to +20 V; thus,V_(g)-I_(d) curves were obtained to examine the temperature dependence.In this embodiment, the temperatures held by the constant-temperaturebath were −30° C., 0° C., 25° C., 40° C., 60° C., 80° C., 100° C., and120° C.

FIG. 5A shows V_(g)-I_(d) curves measured at the above temperatures, andFIG. 5B shows an enlarged view of the portion surrounded by the dottedline in FIG. 5A. The curve indicated by the right arrow is a curvemeasured at −30° C., and the curve indicated by the left arrow is acurve measured at 120° C. The other curves measured at the othertemperatures are positioned therebetween. There is almost no temperaturedependence of on-state current. The off-state current is less than orequal to 1×10⁻¹² [A], which is close to the resolution of themeasurement apparatus, until the gate voltage is lowered to about −20 Vat each temperature; therefore, there is almost no temperaturedependence. That is, it is found that the off-state current of thistransistor is extremely small in view of the off-state current kept lessthan or equal to 1×10⁻¹² [A] and a channel width W of 10000 μm even at ahigh temperature of 120° C.

A transistor including a highly-purified oxide semiconductor (purifiedOS) shows almost no temperature dependence of off-state current. Becausethe oxide semiconductor is highly purified, the conductivity becomes asclose to that of an intrinsic semiconductor as possible, and the Fermilevel is located at the center of a forbidden band; as a result,temperature dependence is not seen. This also results from the fact thatthe oxide semiconductor has an energy gap of 3 eV or more and includesvery few thermally excited carriers. In addition, the source region andthe drain region are in a degenerated state, which is also a factor forshowing no temperature dependence. The transistor is mainly operatedwith carriers which are injected from the degenerated source region tothe oxide semiconductor, and the above characteristics (no temperaturedependence of off-state current) can be explained by no temperaturedependence of carrier density.

In the case where a memory circuit (a memory element) or the like ismanufactured using a transistor having such an extremely small value ofoff-state current, the off-state current has a small value and there isvery little leakage. Therefore, memory data can be held for a longerperiod of time.

Next, a semiconductor device to which the above-described transistor isapplied is described.

FIG. 6 illustrates an example of a block diagram of a memory circuit.The memory circuit illustrated in FIG. 6 includes a row decoder 152, awriting and refresh circuit 154, a column decoder 156, a writing andrefresh circuit 158, and a plurality of memory elements 150 arranged inmatrix. A signal line connected to the memory elements 150 arranged inmatrix is connected to the row decoder 152 through the writing andrefresh circuit 154, and a scan line connected to the memory elements150 arranged in matrix is connected to the column decoder 156 throughthe writing and refresh circuit 158. A bit signal is input to the rowdecoder 152. A read enable signal and a write enable signal (RE/WE), adata signal (data), and an output signal (OUT) are input to the writingand refresh circuit 154 and output from the writing and refresh circuit154.

Each of the memory elements 150 includes a capacitor and a transistor.One of a source and a drain of the transistor is connected to the signalline, and the other of the source and the drain of the transistor isconnected to one electrode of the capacitor, and the other electrode ofthe capacitor is connected to a wiring of the low potential side(preferably, a reference potential Vss).

FIG. 7 illustrates a specific example of the refresh circuit that can beprovided in the writing and refresh circuit 154 illustrated in FIG. 6.Note that the writing and refresh circuit 158 can be formed to have asimilar structure.

The writing and refresh circuit illustrated in FIG. 7 includes a logicalconjunction circuit (an AND circuit) and a sense amplifier 172. To oneinput of each of a first AND circuit 160, a second AND circuit 162, anda third AND circuit 164, a signal is input from the row decoder 152. APRC signal is input to the other input of the first AND circuit 160, thewrite enable signal (WE) is input to the other input of the second ANDcircuit 162, and the read enable signal (RE) is input to the other inputof the third AND circuit 164. The output of the first AND circuit 160controls on/off of a first switch 166, the output of the second ANDcircuit 162 controls on/off of a second switch 168, and the output ofthe third AND circuit 164 controls on/off of a third switch 170. Apre-charge signal line Vprc is connected to the signal line through thefirst switch 166, and a data signal line (data) is connected to thesignal line through the second switch 168.

The signal line connected through the first switch 166 and the secondswitch 168 are connected to the sense amplifier 172 through the thirdswitch 170. A signal is output to the output signal line (OUT) from thesense amplifier 172.

Note that the above AND circuit may have a general structure, andpreferably has a simple structure.

A sense amplifier is a circuit having a function of amplifying inputsignals.

Note that as a signal here, an analog signal or a digital signal whichuses voltage, current, resistance, frequency, or the like can be used,for example. For example, at least two potentials, that is, a firstpotential and a second potential are set, a high-level (also referred toas high potential or V_(H)) potential is used as the first potential,and a low-level (also referred to as low potential or V_(L)) potentialis used as the second potential, whereby a binary digital signal can beset. Although V_(H) and V_(L) are preferably constant values, V_(H) andV_(L) may take a wide range of values, in consideration of influence ofnoise.

Thus, a memory (DRAM) circuit can be manufactured using theabove-described transistors.

A refresh timing of the memory circuit may be determined to a certaintime interval in the design phase, on the basis of the leakage currentof the memory elements 150 which is evaluated in advance. The refreshtiming is set in consideration of the temperature dependence of leakagecurrent and variation of the manufacturing process, after the chip of amemory circuit is completed.

The above-described transistors show almost no temperature dependence ofoff-state current and can keep an extremely small off-state currentvalue. Therefore, when the above-described transistors are used, refreshinterval can be set long as compared with a transistor using silicon,and power consumption in a standby period can be reduced.

Note that the memory circuit used here is not limited to theabove-described DRAM. For example, the memory element may be an SRAM.

FIG. 8 illustrates an example of a circuit configuration of an SRAM inwhich six transistors are provided for one memory element. Note thatalthough FIG. 8 illustrates only one memory element, the number ofmemory elements is not limited to this. A memory element 180 of the SRAMillustrated in FIG. 8 includes an inverter circuit that includes atransistor 186 and a transistor 188, an inverter circuit that includes atransistor 190 and a transistor 192, a transistor 182 and a transistor184 which are operated as switching transistors.

Writing operation of the SRAM illustrated in FIG. 8 is described. When aspecific scan line becomes V_(H) by a column decoder, the transistor 182and the transistor 184 are turned on, and data are written to a pair ofinverter circuits (the inverter circuit including the transistor 186 andthe transistor 188 and the inverter circuit including the transistor 190and the transistor 192). When writing of data is finished, thetransistor 182 and the transistor 184 are turned off, and the datawritten to the pair of inverter circuits are held.

Next, reading operation of the SRAM illustrated in FIG. 8 is described.First, a first signal line and a second signal line are precharged to aspecific potential (a precharge potential) from an external circuit ofthe memory element array. This precharge potential may be set to a valuenear the middle of Vdd and Vss. Each of the first and second signallines having the precharge potential is in a floating state. After that,when the scan line becomes V_(H), the transistor 182 and the transistor184 are turned on and the first signal line and the second signal lineare driven inversely by the pair of inverter circuits. The potentialdifference therebetween is detected by a sense amplifier, so that dataare read out.

The above-described transistors show almost no temperature dependence ofoff-state current and can keep an extremely small off-state currentvalue. Therefore, when the above-described transistors are used, refreshinterval can be set long as compared with a transistor using silicon,and power consumption in a standby period can be reduced.

In the case of using an SRAM as a memory, the number of transistorsformed is not limited to six.

FIG. 9 illustrates an example of a circuit configuration of an SRAM inwhich four transistors are provided for one memory element. In FIG. 9, aresistor 194 and a resistor 196 substitute for the transistor 186 andthe transistor 190 of the SRAM illustrated in FIG. 8, respectively.

Even when the SRAM has the circuit configuration illustrated in FIG. 9,refresh interval can be set long as compared with a transistor usingsilicon, and power consumption in a standby period can be reduced.

As described above, the semiconductor device which is one embodiment ofthe present invention can be obtained.

Embodiment 2

In this embodiment, a transistor that can be used for the semiconductordevice described in Embodiment 1 and is different from the transistor inEmbodiment 1 is described.

FIGS. 10A to 10D illustrate cross-sectional structures of a transistorof this embodiment. A transistor 220 illustrated in FIG. 10D is one modeof a transistor having a bottom-gate structure called achannel-protective type transistor (or a channel-stop type transistor).A process for manufacturing the transistor 220 over a substrate 200 isdescribed below.

First, a conductive film is formed over the substrate 200 having aninsulating surface, and a first photolithography step is performed, sothat the conductive film is processed into a gate electrode layer 202.

The gate electrode layer 202 may be formed using a material similar tothat for the gate electrode layer 111 and may have a single-layerstructure or a stacked-layer structure of a plurality of layers.

Note that a resist mask may be formed by an inkjet method. In the caseof forming a resist mask by an inkjet method, the manufacturing cost canbe reduced because a photomask is not used.

Next, a gate insulating layer 204 is formed so as to cover the gateelectrode layer 202. In this embodiment, a silicon oxynitride layer maybe formed by a plasma CVD method as the gate insulating layer 204.

Next, an oxide semiconductor film is formed over the gate insulatinglayer 204, and then, a second photolithography step is performed, sothat the oxide semiconductor film is processed into an island-shapedoxide semiconductor layer. The oxide semiconductor film may be formed toa thickness of more than or equal to 2 nm and less than or equal to 200nm, for example In this embodiment, for example, the oxide semiconductorfilm is formed by a sputtering method with use of an In—Ga—Zn—O-basedoxide semiconductor target. At this time, it is preferable to performsputtering in the state where moisture remaining in a treatment chamberis removed. This is for preventing hydrogen, hydroxy groups, or moisturefrom being contained in the oxide semiconductor film. Note that, asdescribed in Embodiment 1, an entrapment vacuum pump is preferably usedfor evacuating the treatment chamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the oxide semiconductor film is formed.

Next, as in Embodiment 1, the oxide semiconductor layer is subjected tofirst heat treatment. Here, the substrate 200 is transferred into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, an oxide semiconductorlayer 206 is obtained (FIG. 10A).

Then, plasma treatment using a gas such as N₂O, N₂, or Ar is preferablyperformed. Through this plasma treatment, water or the like adsorbed ona surface of the oxide semiconductor layer 206 which is exposed isremoved. Alternatively, plasma treatment may be performed using a mixedgas of O₂ and Ar.

Next, after an insulating oxide film is formed over the gate insulatinglayer 204 and the oxide semiconductor layer 206, a thirdphotolithography step is performed, so that an insulating oxide layer208 is formed, and a resist mask is removed.

In this embodiment, for example, a 200-nm-thick silicon oxide film isformed by a sputtering method as an insulating oxide film to be theinsulating oxide layer 208. The substrate temperature in film formationmay be more than or equal to room temperature and less than or equal to300° C., and is 100° C. in this embodiment. The sputtering method may beperformed in a rare gas (e.g., argon) atmosphere, an oxygen atmosphere,or an atmosphere of a mixed gas of a rare gas (e.g., argon) and oxygen.Further, a silicon oxide target or a silicon target can be used as atarget, for example. For example, with the use of a silicon target, asilicon oxide film can be formed by a sputtering method in a mixedatmosphere of oxygen and nitrogen. As the insulating oxide layer 208which is formed in contact with the oxide semiconductor layer whoseresistance is reduced, an inorganic insulating film which does notcontain impurities such as moisture, hydrogen ions, and hydroxy groupsand blocks entry of these from the outside is preferably used; forexample, a silicon oxide film, a silicon oxynitride film, an aluminumoxide film, an aluminum oxynitride film, or the like is used. At thistime, it is preferable to perform sputtering in the state where moistureremaining in a treatment chamber is removed. This is for preventinghydrogen, hydroxy groups, or moisture from being contained in the oxidesemiconductor layer 206 and the insulating oxide layer 208. Therefore,in formation of the insulating oxide film, it is preferable to use anentrapment vacuum pump and a high-purity gas from which impurities suchas hydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the insulating oxide film is formed.

Here, second heat treatment may be performed.

The temperature of the second heat treatment is preferably more than orequal to 200° C. and less than or equal to 400° C., and more preferablymore than or equal to 250° C. and less than or equal to 350° C. Notethat the second heat treatment may be performed in an inert gasatmosphere or an oxygen gas atmosphere. In this embodiment, heattreatment may be performed in a nitrogen atmosphere at 250° C. for onehour. In the second heat treatment, heat is applied in the state wherepart of the oxide semiconductor layer 206 (the channel formation region)is in contact with the insulating oxide layer 208.

Heat treatment after the second heat treatment may further be performed.In this embodiment, heat treatment is performed in a nitrogen gasatmosphere, an inert gas atmosphere, or under a reduced pressure in thestate where the insulating oxide layer 208 is formed over the oxidesemiconductor layer 206 which is partly exposed. By the heat treatmentin a nitrogen gas atmosphere or an inert gas atmosphere or under areduced pressure, the resistance of the exposed region of the oxidesemiconductor layer 206, which is not covered with the insulating oxidelayer 208 can be reduced. In this embodiment, for example, heattreatment is performed at 250° C. for 1 hour in a nitrogen atmosphere.By performing the heat treatment in this manner on the oxidesemiconductor layer 206 over which the insulating oxide layer 208 isformed, the resistance of the exposed region of the oxide semiconductorlayer 206 is reduced and the oxide semiconductor layer 206 becomes anoxide semiconductor layer 210 that includes regions having differentresistance. In FIG. 10B, white regions in the oxide semiconductor layer210 without oblique lines are low-resistance regions.

Through the above process up to and including the second heat treatment,heat treatment for dehydration or dehydrogenation is performed on thedeposited oxide semiconductor film to reduce the resistance, and then,part of the oxide semiconductor film is selectively made in anoxygen-excess state. As a result, the channel formation regionoverlapping with the gate electrode layer 202 becomes an i-typesemiconductor having a low conductivity, and a high-resistance sourceregion overlapping with the source electrode layer and a high-resistancedrain region overlapping with the drain electrode layer are formed in aself-aligned manner.

Next, after a conductive film is formed over the gate insulating layer204, the oxide semiconductor layer 210, and the insulating oxide layer208, a fourth photolithography step is performed, so that source anddrain electrode layers 212 are formed, and then a resist mask is removed(FIG. 10C).

The source and drain electrode layers 212 may be formed using a materialsimilar to that for the source electrode layer 115 a and the drainelectrode layer 115 b and may have a single-layer structure or astacked-layer structure of a plurality of layers.

Here, heat treatment may further be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. In this embodiment,the heat treatment is performed at 150° C. for 10 hours. This heattreatment may be performed at a fixed heating temperature, or may beperformed by repeating increasing a temperature from room temperature toa heat temperature of more than or equal to 100° C. and less than orequal to 200° C. and decreasing the temperature from the heattemperature to room temperature plural times. Further, this heattreatment may be performed before formation of the insulating oxidelayer under a reduced pressure. When the heat treatment is performedunder a reduced pressure, the heating time can be shortened. With thisheat treatment, a normally-off transistor can be obtained.

By the formation of the high-resistance drain region (or thehigh-resistance source region) in part of the oxide semiconductor layer,which overlaps with the drain electrode layer (and the source electrodelayer), the reliability of the transistor can be improved. Specifically,by forming the high-resistant drain region, it is possible to employ astructure in which conductivity can be gradually varied from the drainelectrode layer to the high-resistant drain region and the channelformation region. Therefore, in the case where the transistor isoperated in the state where the drain electrode layer is connected to awiring for supplying a high power supply potential Vdd, even when adifference between potentials of the gate electrode layer and the drainelectrode layer is large, the high-resistance drain region serves as abuffer, so that the withstand voltage of the transistor can beincreased.

Through the above-described process, the transistor 220 is formed.

Next, a protective insulating layer 214 is formed over the insulatingoxide layer 208 and the source and drain electrode layers 212. Here, asilicon nitride film may be formed as the protective insulating layer214 (FIG. 10D).

Note that an insulating oxide layer may further be formed over theinsulating oxide layer 208 and the source and drain electrode layers212, and the protective insulating layer 214 may be formed over theinsulating oxide layer.

Although not illustrated, the transistor 220 may be a multi-gatetransistor having a plurality of channel formation regions by forming aplurality of gate electrodes.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 3

In this embodiment, a transistor that can be used for the semiconductordevice described in Embodiment 1 and is different from the transistor inEmbodiment 1 and Embodiment 2 is described.

FIGS. 11A to 11D illustrate cross-sectional structures of a transistorof this embodiment. A transistor 320 illustrated in FIG. 11D is one modeof a transistor having a bottom-gate structure. A process formanufacturing the transistor 320 over a substrate 300 is describedbelow.

First, a conductive film is formed over the substrate 300 having aninsulating surface, and a first photolithography step is performed, sothat the conductive film is processed into a gate electrode layer 302.

The gate electrode layer 302 may be formed using a material similar tothat for the gate electrode layer 111 and may have a single-layerstructure or a stacked-layer structure of a plurality of layers.

Note that a resist mask may be formed by an inkjet method. In the caseof forming a resist mask by an inkjet method, the manufacturing cost canbe reduced because a photomask is not used.

Next, a gate insulating layer 304 is formed so as to cover the gateelectrode layer 302. In this embodiment, a silicon oxynitride layer maybe formed by a plasma CVD method as the gate insulating layer 304.

Next, after a conductive film is formed over the gate insulating layer304, a second photolithography step is performed, so that source anddrain electrode layers 306 a and 306 b are formed (FIG. 11A).

The source and drain electrode layers 306 a and 306 b may be formedusing a material similar to that for the source electrode layer 115 aand the drain electrode layer 115 b and may have a single-layerstructure or a stacked-layer structure of a plurality of layers.

Next, an oxide semiconductor film 308 is formed over the gate insulatinglayer 304 and the source and drain electrode layers 306 a and 306 b(FIG. 11B). Next, the oxide semiconductor film 308 is processed into anisland-shaped oxide semiconductor layer in a third photolithographystep. At this time, it is preferable to perform sputtering in the statewhere moisture remaining in a treatment chamber is removed as inEmbodiment 1. This is for preventing hydrogen, hydroxy groups, ormoisture from being contained in the oxide semiconductor film 308. Notethat, as described in Embodiment 1, an entrapment vacuum pump ispreferably used for evacuating the treatment chamber.

Next, as in Embodiment 1, the oxide semiconductor layer is subjected tofirst heat treatment. Here, the substrate 300 is transferred into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, an oxide semiconductorlayer 310 is obtained (FIG. 11C).

An insulating oxide layer 314 which serves as a protective insulatingfilm and is in contact with the oxide semiconductor layer 310 is formed.The insulating oxide layer 314 is formed to a thickness of more than orequal to 1 nm preferably by a method with which impurities such as wateror hydrogen does not enter the insulating oxide layer 314 in a mannersimilar to that for forming the above-described oxide semiconductorfilm. When hydrogen is contained in the insulating oxide layer 314,entry of the hydrogen to the oxide semiconductor layer or extraction ofoxygen in the oxide semiconductor layer by the hydrogen is caused,whereby part of the oxide semiconductor layer 310 that is in contactwith the insulating oxide layer 314 comes to be n-type (to have a lowerresistance) and thus a parasitic channel might be formed. Therefore, itis important that the insulating oxide layer 314 is formed containing aslittle hydrogen as possible.

The insulating oxide layer 314 can be formed in a manner similar to thatof the insulating oxide layer 116.

Next, second heat treatment is performed. The second heat treatment canbe performed in a manner similar to that in Embodiment 1. In the secondheat treatment, heat is applied in the state where part of the oxidesemiconductor layer (the channel formation region) is in contact withthe insulating oxide layer 314.

Through the above process, heat treatment for dehydration ordehydrogenation is performed on the deposited oxide semiconductor filmto reduce the resistance, and then, the entire surface of the oxidesemiconductor film is selectively made to contain excessive oxygen. As aresult, an i-type oxide semiconductor layer 312 is formed.

Through the above-described process, the transistor 320 is formed.

Here, heat treatment may further be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. In this embodiment,the heat treatment is performed at 150° C. for 10 hours. This heattreatment may be performed at a fixed heating temperature, or may beperformed by repeating increasing a temperature from room temperature toa heat temperature of more than or equal to 100° C. and less than orequal to 200° C. and decreasing the temperature from the heattemperature to room temperature plural times. Further, this heattreatment may be performed before formation of the insulating oxidelayer under a reduced pressure. When the heat treatment is performedunder a reduced pressure, the heating time can be shortened. With thisheat treatment, a normally-off transistor can be obtained.

A protective insulating layer 316 which is similar to the protectiveinsulating layer 103 may be additionally formed over the insulatingoxide layer 314. For example, a silicon nitride film is formed as theprotective insulating layer 316 by an RF sputtering method (FIG. 11D).

A planarization insulating layer for planarization may be provided overthe protective insulating layer 316.

Although not illustrated, the transistor 320 may be a multi-gatetransistor having a plurality of channel formation regions by forming aplurality of gate electrodes.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, a transistor that can be used for the semiconductordevice described in Embodiment 1 and is different from the transistor inany of Embodiments 1 to 3 is described.

FIG. 12A illustrates an example of a plan view of a transistor, and FIG.12B illustrates a cross-sectional view thereof taken along C1-C2 in FIG.12A. A transistor 410 illustrated in FIGS. 12A and 12B is one oftop-gate transistors.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, an insulating layer 407, an oxide semiconductor layer 412,source and drain electrode layers 415 a and 415 b, a gate insulatinglayer 402, and a gate electrode layer 411. A wiring layer 414 a and awiring layer 414 b are provided so as to be in contact with andelectrically connected to the source and drain electrode layers 415 aand 415 b, respectively.

A process for manufacturing the transistor 410 over a substrate 400 isdescribed below.

First, the insulating layer 407 which serves as a base film is formedover the substrate 400 having an insulating surface. As the insulatinglayer 407, an insulating oxide layer such as a silicon oxide layer, asilicon oxynitride layer, an aluminum oxide layer, or an aluminumoxynitride layer is preferably used. Although a plasma CVD method, asputtering method, or the like can be employed as a method for formingthe insulating layer 407, the insulating layer 407 is preferably formedwith a sputtering method so that impurities such as hydrogen iscontained in the insulating layer 407 as little as possible.

In this embodiment, a silicon oxide layer is formed as the insulatinglayer 407 with a sputtering method. The substrate 400 is transferredinto a treatment chamber and a sputtering gas from which hydrogen andmoisture are removed and which contains high-purity oxygen isintroduced, whereby a silicon oxide layer is formed with the use of asilicon target or a quartz target. The substrate 400 may be at roomtemperature or may be heated.

At this time, it is preferable to perform sputtering in the state wheremoisture remaining in a treatment chamber is removed. This is forpreventing hydrogen, hydroxy groups, or moisture from being contained inthe insulating layer 407. Note that, as described in Embodiment 1, anentrapment vacuum pump is preferably used for evacuating the treatmentchamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the insulating layer 407 is formed.

Further, the insulating layer 407 may have a stacked-layer structure inwhich for example, an insulating nitride layer such as a silicon nitridelayer, a silicon nitride oxide layer, an aluminum nitride layer, or analuminum nitride oxide layer and an insulating oxide layer are stackedin this order over the substrate 400.

For example, a sputtering gas from which hydrogen and moisture areremoved and which contains high-purity nitrogen is introduced into thetreatment chamber and a silicon target is used, whereby a siliconnitride layer is formed over a substrate, and a silicon oxide layer isformed thereover. In this case, the silicon nitride layer is preferablyformed in the state where moisture remaining in a treatment chamber isremoved in a manner similar to that for the silicon oxide layer.

In the case where the stack of the silicon nitride layer and the siliconoxide layer is provided as the insulating layer 407, the silicon nitridelayer and the silicon oxide layer can be formed with the use of a commonsilicon target in the same treatment chamber. After a sputtering gascontaining nitrogen is introduced into the treatment chamber first, asilicon nitride layer is formed using a silicon target in the treatmentchamber, and then, the sputtering gas is switched to a sputtering gascontaining oxygen and the same silicon target is used to form a siliconoxide layer. Since the silicon nitride layer and the silicon oxide layercan be formed successively without being exposed to the air in suchsputtering, impurities such as hydrogen and moisture can be preventedfrom adsorbing onto a surface of the silicon nitride layer.

Next, an oxide semiconductor film is formed over the insulating layer407. The oxide semiconductor film is preferably formed to a thickness ofmore than or equal to 2 nm and less than or equal to 200 nm by asputtering method.

The substrate 400 over which the insulating layer 407 is formed ispreferably preheated in a preheat chamber of a sputtering apparatus aspretreatment, and impurities such as hydrogen or moisture adsorbed onthe substrate 400 are preferably eliminated and evacuated, in order thatthe oxide semiconductor film contain as little hydrogen, hydroxy groups,and moisture as possible. The preheat chamber is preferably providedwith a cryopump and evacuated with the cryopump. Note that thispretreatment of preheating may be performed on the substrate 400 beforeforming the gate insulating layer 402 or the substrate 400 over whichthe layers up to and including the source and drain electrode layers 415a and 415 b are formed, in a similar manner.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the oxide semiconductor film is formed.

A sputtering gas from which hydrogen and moisture are removed isintroduced into the treatment chamber in the state where moistureremaining in the treatment chamber is removed, and the oxidesemiconductor film is formed with the use of a metal oxide as a target.Note that an entrapment vacuum pump is preferably used for evacuatingthe treatment chamber.

Next, a first photolithography step is performed, so that the oxidesemiconductor film is processed into an island-shaped oxidesemiconductor layer. A resist mask for forming the island-shaped oxidesemiconductor layer may be formed by an inkjet method.

In the case of forming a resist mask by an inkjet method, themanufacturing cost can be reduced because a photomask is not used.

Note that the etching of the oxide semiconductor film here may beperformed by dry etching, wet etching, or both wet etching and dryetching in combination.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused.

As an etchant used for wet etching, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, an ammonia hydrogenperoxide mixture (hydrogen peroxide solution at 31 wt %: ammonia waterat 28 wt %: water=5:2:2), or the like can be used. Specifically, ITO07N(produced by Kanto Chemical Co., Inc.), which is commercially availableas an etchant for a transparent conductive film from Kanto Chemical Co.,Inc., may also be used.

The etchant after the wet etching is removed together with theetched-off materials by cleaning. The waste liquid including the etchantand the material etched off may be purified and the material may bereused. Materials such as indium contained in the oxide semiconductorlayer are collected from the waste liquid after the etching andrecycled, so that resources can be effectively used and cost can bereduced.

In this embodiment, the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer with a wet etching method with amixed solution of phosphoric acid, acetic acid, and nitric acid as anetchant.

Next, as in Embodiment 1, the oxide semiconductor layer is subjected tofirst heat treatment. Here, the substrate 400 is transferred into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, an oxide semiconductorlayer 412 is obtained (FIG. 13A).

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer. In that case, after thefirst heat treatment, the substrate is taken out of the heat treatmentapparatus, and then a photolithography step is performed.

The heat treatment having effects of dehydration or dehydrogenation ofthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode and a drain electrode are formed over the oxide semiconductorlayer; and after a gate insulating layer is formed over the sourceelectrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 407 and theoxide semiconductor layer 412. The conductive film may be formed in amanner similar to that for the conductive film for the source electrodelayer 115 a and drain electrode layer 115 b.

Next, a second photolithography process is performed. A resist mask isformed over the conductive film and the conductive film is processed, sothat the source and drain electrode layers 415 a and 415 b are formed,and then, the resist mask is removed (FIG. 13B). Note that the sourceelectrode layer and the drain electrode layer preferably have taperedshapes because coverage with the gate insulating layer stacked thereovercan be improved.

In this embodiment, a Ti film is used as the conductive film, and anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 412. As an etchant, an ammonia hydrogen peroxide mixture (a mixedsolution of ammonia, water, and a hydrogen peroxide solution) is used.

In the second photolithography step, only part of the oxidesemiconductor layer 412 is etched off, and an oxide semiconductor layerhaving a groove (a depressed portion) is formed in some cases. A resistmask for forming the source electrode layer 415 a and the drainelectrode layer 415 b may be formed by an inkjet method. In the case offorming a resist mask by an inkjet method, the manufacturing cost can bereduced because a photomask is not used.

Ultraviolet light, KrF laser light, or ArF laser light is preferablyused for light exposure for forming the resist mask in the secondphotolithography step. Note that in the case where light exposure isperformed so that the channel length L becomes less than 25 nm, lightexposure is preferably performed using extreme ultraviolet with anextremely short wavelength of several nanometers to several tens ofnanometers.

Next, a gate insulating layer 402 is formed over the insulating layer407, the oxide semiconductor layer 412, the source electrode layer 415a, and the drain electrode layer 415 b (FIG. 13C).

The gate insulating layer 402 can be formed in a manner similar to thatfor the gate insulating layer 102.

Next, a third photolithography step is performed to form a resist mask,and etching is selectively performed to remove part of the gateinsulating layer 402, so that openings 421 a and 421 b reaching thesource electrode layer 415 a and the drain electrode layer 415 b areformed (FIG. 13D).

Then, after a conductive film is formed over the gate insulating layer402 and in the openings 421 a and 421 b, the gate electrode layer 411and the wiring layers 414 a and 414 b are formed in a fourthphotolithography process. Note that a resist mask may be formed by aninkjet method. In the case of forming a resist mask by an inkjet method,the manufacturing cost can be reduced because a photomask is not used.

The gate electrode layer 411, the wiring layer 414 a and 414 b can beformed in a manner similar to that of the gate electrode layer 111.

Note that the gate electrode layer 411 can also be formed using alight-transmitting conductive film. As an example of a material of thelight-transmitting conductive film, a light-transmitting conductiveoxide or the like can be given.

Next, second heat treatment may be performed.

The temperature of the second heat treatment is preferably more than orequal to 200° C. and less than or equal to 400° C., and more preferablymore than or equal to 250° C. and less than or equal to 350° C. Notethat the second heat treatment may be performed in an inert gasatmosphere or an oxygen gas atmosphere. In this embodiment, the secondheat treatment is performed in a nitrogen atmosphere at 250° C. for onehour. The second heat treatment may be performed after a protectiveinsulating layer or a planarization insulating layer is formed over thetransistor 410.

Here, heat treatment may further be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. In this embodiment,the heat treatment is performed at 150° C. for 10 hours. This heattreatment may be performed at a fixed heating temperature, or may beperformed by repeating increasing a temperature from room temperature toa heat temperature of more than or equal to 100° C. and less than orequal to 200° C. and decreasing the temperature from the heattemperature to room temperature plural times. Further, this heattreatment may be performed before formation of the insulating oxidelayer under a reduced pressure. When the heat treatment is performedunder a reduced pressure, the heating time can be shortened. With thisheat treatment, a normally-off transistor can be obtained.

Through the above-described process, the transistor 410 can be formed(FIG. 13E).

A protective insulating layer or a planarization insulating layer forplanarization may be provided over the transistor 410. For example, theprotective insulating layer may be formed to have a single-layerstructure or a stacked-layer structure using one or more of a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, and an aluminum oxide layer.

By removing moisture included in a treatment chamber when the oxidesemiconductor film is formed in the above manner, the concentration ofhydrogen and that of hydrides contained in the oxide semiconductor filmcan be reduced.

Although not illustrated, the transistor 410 may be a multi-gatetransistor having a plurality of channel formation regions by forming aplurality of gate electrodes.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 5

In this embodiment, a transistor that can be used for the semiconductordevice described in Embodiment 1 and is different from the transistor inany of Embodiments 1 to 4 is described.

FIG. 14A illustrates a cross-sectional structure of a transistor of thisembodiment. A transistor 520 illustrated in FIG. 14A includes aninsulating layer 522 and a conductive layer 527 interposed between thesubstrate 400 and the insulating layer 407 of the transistor 410illustrated in FIG. 13E. The conductive layer 527 overlaps with theentire oxide semiconductor layer 512. A transistor 521 illustrated inFIG. 14B includes the insulating layer 522 and a conductive layer 524interposed between the substrate 400 and the insulating layer 407 of thetransistor 410 illustrated in FIG. 13E. The conductive layer 524overlaps with part of the oxide semiconductor layer 512 (specifically, achannel formation region).

The conductive layer 524 and the conductive layer 527 may be formedusing the same material and method as the gate electrode layers of thetransistor 520 and the transistor 521, or may be formed using differentmaterials and methods. The conductive layer 524 and the conductive layer527 can function as a second gate electrode layer. Potentials of theconductive layer 524 and the conductive layer 527 may each be a fixedpotential of GND or 0 V.

It is possible to control electric characteristics of the transistor 520and the transistor 521 (e.g., threshold voltages) by the conductivelayer 524 and the conductive layer 527.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 6

In this embodiment, a transistor that can be used for the semiconductordevice described in Embodiment 1 and is different from the transistor inany of Embodiments 1 to 5 is described.

FIG. 15A illustrates a plan view of a transistor 610 having a top-gatestructure of this embodiment, and FIG. 15B illustrates a cross-sectionalview thereof taken along D1-D2 in FIG. 15A.

The transistor 610 includes, over a substrate 600 having an insulatingsurface, an insulating layer 607, source and drain electrode layer 615 a(615 a 1 and 615 a 2), an oxide semiconductor layer 612, source anddrain electrode layer 615 b, a wiring layer 618, a gate insulating layer602, and a gate electrode layer 611 (611 a and 611 b). The source anddrain electrode layer 615 a (615 a 1 and 615 a 2) are electricallyconnected to the wiring layer 614 through the wiring layer 618. Althoughnot illustrated, the source and drain electrode layer 615 b iselectrically connected to the wiring layer 614 in an opening formed inthe gate insulating layer 602.

A process for manufacturing the transistor 610 over a substrate 600 isdescribed below with reference to FIGS. 16A to 16E.

First, the insulating layer 607 which serves as a base film is formedover the substrate 600 having an insulating surface.

In this embodiment, a silicon oxide layer is formed as the insulatinglayer 607 with a sputtering method. The substrate 600 is transferredinto a treatment chamber and a sputtering gas from which hydrogen andmoisture are removed and which contains high-purity oxygen isintroduced, whereby a silicon oxide layer is formed with the use of asilicon target or a quartz target as the insulating layer 607 over thesubstrate 600. Note that oxygen or a mixed gas of oxygen and argon isused as a sputtering gas.

At this time, it is preferable to perform sputtering in the state wheremoisture remaining in a treatment chamber is removed. This is forpreventing hydrogen, hydroxy groups, or moisture from being contained inthe insulating layer 607. Note that, as described in Embodiment 1, anentrapment vacuum pump is preferably used for evacuating the treatmentchamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the insulating layer 607 is formed.

Further, the insulating layer 607 may have a stacked-layer structure inwhich for example, an insulating nitride layer such as a silicon nitridelayer, a silicon nitride oxide layer, an aluminum nitride layer, or analuminum nitride oxide layer and an oxide insulating layer are stackedin this order over the substrate 600.

For example, a sputtering gas from which hydrogen and moisture areremoved and which contains high-purity nitrogen is introduced into thetreatment chamber and a silicon target is used, whereby a siliconnitride layer is formed over a substrate, and a silicon oxide layer isformed thereover. In this case, the silicon nitride layer is preferablyformed in the state where moisture remaining in a treatment chamber isremoved in a manner similar to that for the silicon oxide layer.

Next, a conductive film is formed over the insulating layer 607 and afirst photolithography process is performed. A resist mask is formedover the conductive film and the conductive film is processed, so thatthe source and drain electrode layers 615 a 1 and 615 a 2 are formed,and then, the resist mask is removed (FIG. 16A). In the cross-sectionalview, the source and drain electrode layers 615 a 1 and 615 a 2 seem tobe separated from each other; however, they are not actually separated.Note that the source and drain electrode layers 615 a 1 and 615 a 2preferably have tapered shapes because coverage with the oxidesemiconductor layer 612 stacked thereover can be improved.

As a material of the source and drain electrode layers 615 a 1 and 615 a2, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloycontaining any of these elements as a component; an alloy filmcontaining any of these elements in combination; and the like can begiven. In addition, one or more of Mn, Mg, Zr, Be, and Th may also beused. The conductive film may have a single-layer structure or astacked-layer structure of a plurality of layers. For example, asingle-layer structure of an Al film containing Si, a two-layerstructure in which a Ti film is stacked over an Al film, a three-layerstructure in which an Al film is interposed between two Ti films, or thelike can be given. When the conductive film includes an Al film, it maycontain Nd or Sc. Note that these films may be nitride films.

In this embodiment, a 150-nm-thick titanium film is formed as the sourceand drain electrode layers 615 a 1 and 615 a 2 by a sputtering method.

Next, an oxide semiconductor film with a thickness more than or equal to2 nm and less than or equal to 200 nm is formed over the insulatinglayer 607 and the source and drain electrode layers 615 a 1 and 615 a 2.

Note that the oxide semiconductor film is preferably formed in the statewhere moisture remaining in the treatment chamber is removed. This isfor preventing hydrogen, hydroxy groups, or moisture from beingcontained in the oxide semiconductor film. An entrapment vacuum pump ispreferably used for evacuating the treatment chamber.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, hydroxy groups, or hydrides are removed to aconcentration of less than or equal to 1 ppm (preferably, aconcentration of less than or equal to 10 ppb) as a sputtering gas usedwhen the oxide semiconductor film is formed.

Next, a second photolithography step is performed, so that the oxidesemiconductor film is processed into an island-shaped oxidesemiconductor layer (FIG. 16B). In this embodiment, the oxidesemiconductor film is formed by a sputtering method with use of anIn—Ga—Zn—O-based oxide semiconductor target.

Next, first heat treatment is performed on the island-shaped oxidesemiconductor layer, so that dehydration or dehydrogenation of the oxidesemiconductor layer is conducted. The temperature of the first heattreatment is more than or equal to 400° C. and less than or equal to750° C., preferably more than or equal to 400° C. and less than thestrain point of the substrate. Here, the substrate 600 is transferredinto an electric furnace which is one of heat treatment apparatuses,heat treatment is performed on the oxide semiconductor layer in anitrogen atmosphere at 450° C. for one hour, and then, the oxidesemiconductor layer is not exposed to the air so that entry of water andhydrogen into the oxide semiconductor layer is prevented; thus, an oxidesemiconductor layer 612 is obtained (FIG. 16B).

The heat treatment apparatus is not limited to the above-describedelectric furnace but may be a heat treatment apparatus provided with aunit that heats an object to be processed by thermal conduction orthermal radiation from a heater such as a resistance heater or the like.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. Note that the GRTA apparatus is a heat treatmentapparatus using a high-temperature gas. As the high-temperature gas, aninert gas which is unlikely to react with an object to be processed byheat treatment, such as nitrogen gas or a rare gas like argon, is used.GRTA enables a high-temperature heat treatment for a short time.

Note that it is preferable that in the first heat treatment, nitrogen ora rare gas such as helium, neon, or argon, which is a gas introducedinto the treatment chamber does not contain water, hydrogen, and thelike. Alternatively, the purity of a nitrogen gas or a rare gas such ashelium, neon, or argon which is introduced into the heat treatmentapparatus is preferably 6N (99.9999%) or higher, more preferably 7N(99.99999%) or higher (that is, the impurity concentration is less thanor equal to 1 ppm, preferably less than or equal to 0.1 ppm).

The first heat treatment of the oxide semiconductor layer can beperformed on the oxide semiconductor film before being processed intothe island-shaped oxide semiconductor layer. In that case, after thefirst heat treatment, the substrate is taken out of the heat treatmentapparatus, and then a photolithography step is performed.

The heat treatment having effects of dehydration or dehydrogenation ofthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode and a drain electrode are further formed over the oxidesemiconductor layer; and after a gate insulating layer is formed overthe source electrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 607 and theoxide semiconductor layer 612, and a third photolithography step isperformed. A resist mask is formed over the conductive film and theconductive film is processed, so that the source and drain electrodelayers 615 b and the wiring layer 618 are formed. After that, the resistmask is removed (FIG. 16C). The source and drain electrode layer 615 band the wiring layer 618 may be formed using a material and stepssimilar to those of the source and drain electrode layers 615 a 1 and615 a 2.

In this embodiment, a 150-nm-thick titanium film is formed as the sourceand drain electrode layer 615 b and the wiring layer 618 by a sputteringmethod. In this embodiment, since the same titanium films are used forthe source and drain electrode layers 615 a 1 and 615 a 2 and the sourceand drain electrode layer 615 b, etching selectivity of the source anddrain electrode layers 615 a 1 and 615 a 2 with respect to the sourceand drain electrode layer 615 b is not obtained. Thus, the wiring layer618 is provided over part of the source and drain electrode layer 615 a2 which is not covered with the oxide semiconductor layer 612 so thatthe source and drain electrode layers 615 a 1 and 615 a 2 are not etchedat the time of etching of the source and drain electrode layer 615 b. Inthe case where high etching selectivity ratio of the source and drainelectrode layers 615 a 1 and 615 a 2 to the source and drain electrodelayer 615 b can be obtained, the wiring layer 618 is not necessarilyprovided.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 612 is not removed inetching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, and anIn—Ga—Zn—O-based oxide semiconductor is used for the oxide semiconductorlayer 612. As an etchant, an ammonia hydrogen peroxide mixture (a mixedsolution of ammonia, water, and a hydrogen peroxide solution) is used.

In the third photolithography step, only part of the oxide semiconductorlayer 612 is etched off, and an oxide semiconductor layer having agroove (a depressed portion) is formed in some cases. A resist mask forforming the source and drain electrode layer 615 b and the wiring layer618 may be formed by an inkjet method. In the case of forming a resistmask by an inkjet method, the manufacturing cost can be reduced becausea photomask is not used.

Next, a gate insulating layer 602 is formed over the insulating layer607, the oxide semiconductor layer 612, the source and drain electrodelayers 615 a 1 and 615 a 2, and the source and drain electrode layer 615b.

The gate insulating layer 602 can be formed to have a single-layerstructure or a stacked-layer structure using one or more of a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, and an aluminum oxide layer by a plasma CVDmethod, a sputtering method, or the like. Note that the insulating layer602 is preferably formed with a sputtering method so that a large amountof hydrogen is not contained in the gate insulating layer 602. In thecase where a silicon oxide film is formed by a sputtering method, asilicon target or a quartz target is used as a target, and oxygen or amixed gas of oxygen and argon is used as a sputtering gas.

The gate insulating layer 602 may have a structure where a silicon oxidelayer and a silicon nitride layer are stacked over the source and drainelectrode layers 615 a 1 and 615 a 2 and the source and drain electrodelayer 615 b. In this embodiment, a silicon oxide layer is formed to athickness of 100 nm with a sputtering method in a mixed gas atmosphereof oxygen and argon.

Next, a fourth photolithography process is performed. A resist mask isformed and selective etching is performed to remove part of the gateinsulating layer 602, so that an opening 623 reaching the wiring layer618 is formed (FIG. 16D). Although not illustrated, in forming theopening 623, an opening reaching the source and drain electrode layer615 b may be formed.

Then, after a conductive film is formed over the gate insulating layer602 and in the opening 623, the gate electrode layer 611 (the gateelectrode layer 611 a and the gate electrode layer 611 b) and the wiringlayer 614 are formed in a fifth photolithography process. Note that aresist mask may be formed by an inkjet method. In the case of forming aresist mask by an inkjet method, the manufacturing cost can be reducedbecause a photomask is not used.

The gate electrode layer 611 (the gate electrode layer 611 a and thegate electrode layer 611 b) and the wiring layer 614 can be formed tohave a single-layer structure or a stacked-layer structure using one ormore of metal materials such as Mo, Ti, Cr, Ta, W, Al, Cu, Nd and Sc,and alloy materials containing the above metal material as its maincomponent.

In this embodiment, a 150-nm-thick titanium film is formed as the gateelectrode layer 611 (the gate electrode layer 611 a and the gateelectrode layer 611 b) and the wiring layer 614 by a sputtering method.

Next, second heat treatment is performed. The temperature of the secondheat treatment is preferably more than or equal to 200° C. and less thanor equal to 400° C., and more preferably more than or equal to 250° C.and less than or equal to 350° C. Note that the second heat treatmentmay be performed in an inert gas atmosphere or an oxygen gas atmosphere.In this embodiment, the second heat treatment is performed in a nitrogenatmosphere at 250° C. for one hour. The second heat treatment isperformed in the state where part of the oxide semiconductor layer (achannel formation region) is in contact with the gate insulating layer602. Note that the second heat treatment may be performed after aprotective insulating layer or a planarization insulating layer isformed over the transistor 610.

Here, heat treatment may further be performed at more than or equal to100° C. and less than or equal to 200° C. for more than or equal to 1hour and less than or equal to 30 hours in the air. This heat treatmentmay be performed at a fixed heating temperature, or may be performed byrepeating increasing a temperature from room temperature to a heattemperature of more than or equal to 100° C. and less than or equal to200° C. and decreasing the temperature from the heat temperature to roomtemperature plural times. Further, this heat treatment may be performedbefore formation of the insulating oxide layer under a reduced pressure.

Through the above process, the transistor 610 including the oxidesemiconductor layer 612 in which the concentration of hydrogen,moisture, hydrides, and hydroxides is reduced can be formed (FIG. 16E).

A protective insulating layer or a planarization insulating layer forplanarization may be provided over the transistor 610. Although notillustrated, an opening reaching the source and drain electrode layer615 b is formed in the gate insulating layer 602, the protectiveinsulating layer, and the planarization insulating layer. A wiring layeris formed in the opening to be electrically connected to the source anddrain electrode layer 615 b.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 7

In this embodiment, a transistor which is one embodiment of the presentinvention whose structure is similar to that of the transistorillustrated in FIG. 14B is described with reference to energy banddiagrams.

FIG. 17 is a longitudinal sectional view of an inverted staggered thinfilm transistor of this embodiment in which an oxide semiconductor isused. An oxide semiconductor film (OS) is provided over a gate electrode(GE1) with a gate insulating film (GI) therebetween, and a sourceelectrode (S) and a drain electrode (D) are provided over the oxidesemiconductor film.

FIGS. 18A and 18B are energy band diagrams (schematic diagrams) on A-A′in FIG. 17. FIG. 18A shows a case where the source and the drain havevoltage of the same potential (V_(d)=0 V). FIG. 18B shows a case wherepositive potential is applied to the drain (V_(d)>0 V) whereas positivepotential is not applied to the source.

FIGS. 19A and 19B are energy band diagrams (schematic diagrams) on B-B′in FIG. 17. FIG. 19A shows a state where a positive potential (+V_(g))is applied to a gate (G1), that is, a case where the thin filmtransistor is in an on-state where carriers (electrons) flow between thesource and the drain. FIG. 19B shows a state where a negative potential(−V_(g)) is applied to the gate (G1), that is, a case where the thinfilm transistor is in an off-state (where minority carriers do notflow).

FIG. 20 shows relation between the vacuum level and the work function ofa metal (ϕ_(M)) and relation between the vacuum level and the electronaffinity of an oxide semiconductor (χ).

Electrons in a metal are degenerated at room temperature, so that theFermi level is located in the conduction band. In contrast, aconventional oxide semiconductor is generally of n-type, and the Fermilevel (E_(F)) in that case is located closer to the conduction band andis away from the intrinsic Fermi level (E_(i)) that is located in themiddle of the band gap. It is known that the n-type semiconductor isformed partly because some of hydrogen in the oxide semiconductor servesas donors.

In contrast, the oxide semiconductor of this embodiment is an oxidesemiconductor that is made to be an intrinsic (i-type) semiconductor ormade to be as close to an intrinsic semiconductor as possible by beinghighly purified by removal of hydrogen that is an n-type impurity sothat as few impurities, which are not main components of the oxidesemiconductor, as possible are contained. In other words, the oxidesemiconductor of this embodiment has a feature in that it is made to bean i-type (intrinsic) semiconductor or made to be close thereto by beinghighly purified by removal of as many impurities such as hydrogen orwater as possible, instead of addition of impurities. As a result, theFermi level (E_(F)) can be the same level as the intrinsic Fermi level(E_(i)).

It is said that the electron affinity (χ) of an oxide semiconductor is4.3 eV in the case where the band gap (E_(g)) thereof is 3.15 eV. Thework function of titanium (Ti) used for forming the source and drainelectrodes is substantially equal to the electron affinity of the oxidesemiconductor (χ). In this case, the Schottky electron barrier is notformed at an interface between the metal and the oxide semiconductor.

In other words, an energy band diagram (a schematic diagram) like FIG.18A is obtained in the case where a metal and an oxide semiconductor arein contact with each other when the work function of the metal (ϕ_(M))and the electron affinity of the oxide semiconductor (χ) aresubstantially equal.

In FIG. 18B, black circles (•) represent electrons. A dashed lineindicates a case where a positive voltage is applied to a gate (V_(g)>0)in the state where a positive potential is given to a drain. When novoltage is applied to the gate, carriers (electrons) are not injectedfrom an electrode to an oxide semiconductor side due to a high potentialbarrier, resulting in an off-state in which no current flows. Incontrast, when a positive voltage is applied to the gate, the potentialbarrier is reduced, resulting in an on-state in which current flows.

In this case, as shown in FIG. 19A, the electron moves along the lowestpart of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the highly-purified oxidesemiconductor.

In FIG. 19B, when a negative potential is applied to the gate (G1), thenumber of holes that are minority carriers is substantially zero; thus,the current value becomes a value extremely close to zero.

For example, even when the thin film transistor has a channel width W of1×10⁴ μm and a channel length of 3 μm, an off-state current of 10⁻¹³ Aor lower and a subthreshold swing (S value) of 0.1 V/dec. (the thicknessof the gate insulating film: 100 nm) can be obtained.

As described above, the oxide semiconductor is highly purified so thatthe amount of impurities that are not main components of the oxidesemiconductor are minimized, whereby favorable operation of the thinfilm transistor can be obtained. In particular, the off-state currentcan be decreased.

Embodiment 8

In this embodiment, described is a central processing unit (CPU) towhich the semiconductor device described in Embodiment 1, which is oneembodiment of the present invention, is applied.

An example of a block diagram of a CPU is illustrated in FIG. 21. A CPU801 illustrated in FIG. 21 includes a timing control circuit 802, aninstruction decoder 803, a register array 804, an address logic andbuffer circuit 805, a data bus interface 806, an ALU (an arithmeticlogic unit) 807, an instruction register 808, and the like.

These circuits included in the CPU 801 can be manufactured using thetransistor described in any of Embodiments 1 to 7. The transistorsdescribed in Embodiments 1 to 7 each use an oxide semiconductor layer,whereby field-effect mobility can be increased. In addition, whenhydrogen concentration is sufficiently reduced, the off-state current ofthe transistor can be extremely small. By using a transistor includingan oxide semiconductor layer with sufficiently reduced hydrogenconcentration for at least part of the CPU 801, the power consumption ofthe central processing unit can be reduced.

Now, each circuit included in the CPU 801 is briefly described.

The timing control circuit 802 receives an instruction from theexternal, converts the instruction into information for the internal,and sends the information to other blocks. In addition, the timingcontrol circuit gives directions such as reading and writing of memorydata to the external, according to internal operation.

The instruction decoder 803 serves to convert instruction from theexternal into instruction for the internal.

The register array 804 has a function of temporarily storing data.Therefore, it includes a memory element.

The address logic and buffer circuit 805 has a function of specifyingthe address of an external memory.

The data bus interface 806 has a function of taking data in and out ofan external memory or a device such as a printer.

The ALU 807 has a function of performing an operation.

The instruction register 808 has a function of temporarily storing aninstruction. Therefore, it includes a memory element.

By using any of the transistors described in Embodiments 1 to 7 for atleast part of the CPU 801, leakage current is reduced; thus, powerconsumption in a standby period (standby power) is reduced. Therefore,power consumption of such central processing unit can be reduced. Thememory element described in Embodiment 1, for example, can beparticularly applied to a memory element included in the register array804 or the command register 808 among circuits included in the CPU 801.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 9

In this embodiment, a semiconductor device which is one embodiment ofthe present invention is described. As an example of a semiconductordevice to which the transistor described in any of Embodiment 1 toEmbodiment 7 is applied, a wireless communication semiconductor devicecapable of input/output data without contact is described. The wirelesscommunication semiconductor device capable of input/output data withoutcontact is also called an RFID tag, an ID tag, an IC tag, an RF tag, awireless tag, an electronic tag, or a wireless chip.

One example of a structure of the wireless communication semiconductordevice of this embodiment is described with reference to FIGS. 22A to22C. The wireless communication semiconductor device illustrated in FIG.22A includes a semiconductor integrated circuit chip 900 having anantenna (also referred to as an on-chip antenna) and a supportingsubstrate 904 having an antenna 902 (also referred to as a boosterantenna). The semiconductor integrated circuit chip 900 is provided overan insulating layer 908 formed over the supporting substrate 904 and theantenna 902. The insulating layer 908 may be formed by a sealant or thelike as long as it can fix the semiconductor integrated circuit chip 900to the supporting substrate 904 and the antenna 902.

Note that a conductive shield is preferably provided on a surface of thesemiconductor integrated circuit chip 900 to prevent electrostaticbreakdown of the semiconductor integrated circuit (e.g., malfunction ofthe circuit and damage to a semiconductor element) due to electrostaticdischarge. Note that when the conductive shield has high resistance andcurrent cannot pass through the pattern of the antenna 902, the antenna902 and the conductive shield provided on the surface of thesemiconductor integrated circuit chip 900 may be provided in contactwith each other.

A semiconductor integrated circuit provided in the semiconductorintegrated circuit chip 900 includes a plurality of elements such astransistors constituting a memory portion or a logic portion. As atransistor for constituting a memory portion or a logic portion, atransistor described in any of Embodiments 1 to 7 can be used.

FIG. 23A is an enlarged view of the antenna (the on-chip antenna) andthe semiconductor integrated circuit included in the semiconductorintegrated circuit chip 900 illustrated in FIG. 22A. Although theantenna 912 is a rectangular loop antenna with one winding in FIG. 23A,the present invention is not limited to this structure. The shape of theantenna 912 may be a shape with curve (e.g., a circle), and the numberof windings of the antenna 912 may be plural. However, the number ofwindings of the antenna 912 is preferably 1, in which case parasiticcapacitance generated between the semiconductor integrated circuit 910and the antenna 912 can be reduced.

In FIG. 23A, the antenna 912 is placed to surround the periphery of thesemiconductor integrated circuit 910, and except for a portioncorresponding to power feeding points 918 indicated by dashed lines, theantenna 912 is arranged in a region that does not overlap with thesemiconductor integrated circuit 910. When the antenna 912 and thesemiconductor integrated circuit 910 are placed in different regions asillustrated in FIG. 23A, the parasitic capacitance between thesemiconductor integrated circuit 910 and the antenna 912 can be reduced.However, the present invention is not limited to this structure. Asshown in FIG. 23B, the antenna 912 may be provided so as to at leastpartly overlap with the semiconductor integrated circuit 910 except forthe portions which correspond to the power feeding points 918.

In FIG. 22A, the antenna 902 can send and receive signals or supplypower to/from the antenna 912 included in the semiconductor integratedcircuit chip 900 by electromagnetic induction mainly in a loop-shapedportion surrounded by a dashed line 906. In addition, the antenna 902can send and receive a signal to/from an external interrogator by usinga radio wave mainly in a region other than a portion surrounded by adashed line 906. In addition to sending and receiving a signal,supplying electric power may be performed by the external interrogator.Although there is no particular limitation, it is preferable that thefrequency of a radio wave used as a carrier (carrier wave) between theinterrogator and the semiconductor integrated circuit chip 900 beapproximately more than or equal to 30 MHz and less than or equal to 5GHz; for example, a frequency band of 950 MHz, 2.45 GHz, or the like maybe employed.

Although the antenna 902 is a rectangular loop antenna in which thenumber of windings is 1 in the portion surrounded by the dashed line906, an embodiment of the present invention is not limited to thisstructure. The shape of the antenna 902 may be a shape with curve (e.g.,a circle). In addition, the number of windings may be plural. However,when the number of windings of the antenna 902 is preferably 1 in theportion surrounded by the dashed line 906, in which case parasiticcapacitance generated between the antenna 902 and the antenna 912 can bereduced.

There is no particular limitation on communication methods of thewireless communication semiconductor device of this embodiment; forexample, an electromagnetic induction method, an electromagneticcoupling method, or a microwave method can be used. In the case of usinga microwave method (for example, using a UHF band (a band of 860 MHz to960 MHz), a 2.45 GHz band, or the like), the length and shape of theantennas 902 and 912 may be decided depending on the wavelength ofelectromagnetic wave used. The antennas can have a linear shape (e.g., adipole antenna), a flat shape (e.g., a patch antenna or an antennahaving a ribbon shape), or the like, instead of the above-describedshapes. The antenna may have a serpentine curved shape, or a shapecombining them instead of the above-described shapes.

The communication method of the wireless communication semiconductordevice of this embodiment may be an electromagnetic induction method oran electromagnetic coupling method, as described above. FIG. 24illustrates an example in which the electromagnetic induction method orthe electromagnetic coupling method is used.

In FIG. 24, the coil antenna 902 is provided as a booster antenna overthe supporting substrate 904, and the semiconductor integrated circuitchip 900 including the coil antenna 912 is provided over the supportingsubstrate 904.

Next, the structures and arrangements of the semiconductor integratedcircuit chip 900 and the booster antenna are described. FIG. 22B is aperspective view of a semiconductor device in which the semiconductorintegrated circuit chip 900 and the antenna 902 provided for the supportsubstrate 904 illustrated in FIG. 22A are stacked. FIG. 22C correspondsto a cross-sectional view taken along dashed line X-Y of FIG. 22B.

The semiconductor integrated circuit 910 illustrated in FIG. 22C is heldbetween a first insulator 914 and a second insulator 916, and the sidesurfaces thereof are also sealed. In this embodiment, a first insulatorand a second insulator between which a plurality of semiconductorintegrated circuits are interposed are attached, and then thesemiconductor integrated circuits may be individually divided intostacks. A conductive shield is formed each for the divided stacks, andthe semiconductor integrated circuit chips 900 are formed. There is noparticular limitation on a separation means as long as physicalseparation is possible, and separation is performed by laser beamirradiation for example.

In FIG. 22C, the semiconductor integrated circuit 910 is closer to theantenna 902 than the antenna 912; however, the present invention is notlimited to this structure. The antenna 912 may be closer to the antenna902 than the semiconductor integrated circuit 910.

Next, operation of the wireless communication semiconductor device ofthis embodiment is briefly described below. FIG. 25 is a block diagramillustrating an example of a structure of the wireless communicationsemiconductor device of this embodiment. The portions that are the sameas those in FIGS. 22A, 22B, and 22C, FIGS. 23A and 23B, and FIG. 24 aredenoted by the same reference numerals. The wireless communicationsemiconductor device illustrated in FIG. 25 includes the antenna 902,the semiconductor integrated circuit 910, and the antenna 912. Theantenna 902 is a booster antenna, and the antenna 912 is an on-chipantenna.

First, the case where the wireless communication semiconductor devicereceives signals and electric power from the interrogator 920 isdescribed. First, when an electromagnetic wave is sent from theinterrogator 920, the antenna 902 receives the electromagnetic wave,whereby alternating current is generated in the antenna 902 and amagnetic field is generated around the antenna 902. Then, a loop-shapedportion included in the antenna 902 and the antenna 912 having a loopshape are electromagnetically coupled, and induced electromotive forceis generated in the antenna 912. The semiconductor integrated circuit910 receives a signal or power from the interrogator 920 by using theinduced electromotive force.

Then, the case where the wireless communication semiconductor devicesends signals to the interrogator 920 is described. At this time,current is made to flow through the antenna 912 and inducedelectromotive force is generated in the antenna 902 in accordance with asignal generated in the semiconductor integrated circuit 910, wherebythe signal can be transmitted to the interrogator 920 as a reflectedwave of a radio wave transmitted from the interrogator 920.

Note that the antenna 902 can mainly be divided into a loop-shapedportion which is coupled electromagnetically with the antenna 912 and aportion which receives a radio wave from the interrogator 920. The shapeof the antenna 902 in the portion in which an electric wave from theinterrogator 920 is mainly received may be a shape in which an electricwave can be received. For example, shapes such as a dipole antenna, afolded-dipole antenna, a slot antenna, a meander line antenna, or amicrostrip antenna may be used.

Although the case where one on-chip antenna and one booster antenna areincluded is described above, there is no limitation to this case. Anantenna for electric power and an antenna for sending and receivingsignals may be separately formed. By separately forming the antenna forelectric power and the antenna for sending and receiving signals, it ispossible to vary the frequency of the radio wave for supplying electricpower and the frequency of the radio wave for sending signals; as aresult, electric power can be supplied efficiently and signals can besent and received efficiently.

In a semiconductor device of this embodiment, the on-chip antenna isused and a signal or power can be sent and received between the boosterantenna and the on-chip antenna without contact; therefore, unlike thecase where a semiconductor integrated circuit is physically in directconnection to an external antenna, the semiconductor integrated circuitand the antenna are less likely to be disconnected due to externalforce, and generation of initial failure in the connection can also besuppressed.

In addition, since the booster antenna is used in the semiconductordevice of this embodiment, unlike the case where only the on-chipantenna is used, the area of the semiconductor integrated circuit doesnot significantly limit the size or shape of the on-chip antenna.Accordingly, there is no limitation on the frequency band of radio wavescapable of being received, and the communication distance can be longerthan that of the semiconductor device using only the on-chip antenna.

The semiconductor integrated circuit of this embodiment may be formedover a flexible substrate. In the case of using a flexible substrate,the semiconductor integrated circuit may be formed directly on theflexible substrate, or may be formed on another substrate such as aglass substrate for formation of the semiconductor integrated circuit,and then transferred to a flexible substrate such as a plasticsubstrate. There is no particular limitation on the method oftransferring the semiconductor integrated circuit from the formationsubstrate to a flexible substrate; for example, a separation layer maybe formed between the formation substrate and the semiconductorintegrated circuit.

In the case where the semiconductor integrated circuit is transferred toa flexible substrate, the separation layer can be formed using, forexample, a metal oxide. In this case, the formed metal oxide is weakenedby crystallization, and an element layer including the semiconductorintegrated circuit, which is a layer to be separated, can be separatedfrom the formation substrate. After the metal oxide film iscrystallized, part of the separation layer may be removed with use of asolution or a halogen fluoride gas such as NF₃, BrF₃, or ClF₃, and thenseparation may be performed.

In addition, when a substrate having a light-transmitting property isused as the formation substrate and a film containing nitrogen, oxygen,hydrogen or the like (e.g., an amorphous silicon film containinghydrogen, an alloy film containing hydrogen, an alloy film containingoxygen or the like) is used as the separation layer, the separationlayer is irradiated with laser light through the formation substrate,and nitrogen, oxygen, or hydrogen contained in the separation layer isevaporated so that separation can occur.

Alternatively, a method of removing the formation substrate mechanically(e.g., grinding) or a method of removing the formation substratechemically (e.g., etching using any of the above-described halogenfluoride gases) may be employed. In this case, the separation layer isnot necessarily used.

Alternatively, laser irradiation, a sharp knife, or the like can be usedso as to form a groove to expose the separation layer and the separationmay be performed by using the groove as a trigger.

When the above-described separation is performed, for example,mechanical force may be applied. As a method for applying the mechanicalforce, for example, a separation process with a human hand or with agripper, a separation process by rotation of a roller, or the like canbe given.

Note that the separation layer can be formed using tungsten, forexample. When the separation layer is formed using tungsten, separationcan be performed while the separation layer is being etched by a mixedsolution of ammonia water and hydrogen peroxide solution.

The transistors described in Embodiments 1 to 7 have a small off-statecurrent; therefore, by applying any of the transistors to thesemiconductor device of this embodiment, low power consumption can berealized.

By the conductive shields covering the semiconductor integrated circuit,electrostatic breakdown of the semiconductor integrated circuit can beprevented.

With a pair of insulators between which the semiconductor integratedcircuit is interposed, a highly reliable semiconductor device whileachieving reduction in thickness and size can be provided.

Accordingly, a highly reliable semiconductor device with low powerconsumption in which electrostatic discharge is suppressed can beobtained.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 10

In this embodiment, examples in each of which the wireless communicationsemiconductor device described in Embodiment 9 is used as one embodimentof the present invention are described.

FIGS. 26A to 26F illustrate application examples of a semiconductordevice 1000 which is similar to the wireless communication semiconductordevice described in Embodiment 9. The semiconductor device 1000 can beused for a variety of items and systems by utilizing a function ofsending and receiving an electromagnetic wave. As the items, thefollowing items are given: keys (see FIG. 26A), paper money, coins,securities, bearer bonds, certificates (such as a driver's license or aresident's card, see FIG. 26B), books, containers (such as a Petri dish,see FIG. 26C), personal accessories (such as bags or eyeglasses, seeFIG. 26D), packaging containers (such as wrapping paper or bottles, seeFIGS. 26E and 26F), recording media (such as a disk or video tape),vehicles (such as a bicycle), food, clothing, livingware, electronicappliances (such as a liquid crystal display device, an EL displaydevice, a television device, or a portable terminal), and the like. Thesemiconductor device 1000 is fixed to such items having a variety ofshapes by being attached to or embedded in a surface. As the systems, agoods management system, an authentication function system, adistribution system, and the like can be given.

By using a highly reliable semiconductor device with low powerconsumption in which electrostatic discharge is suppressed, a highlyreliable system can be obtained.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 11

In this embodiment, described is a display device to which thetransistor described in any one of Embodiments 1 to 7 is applied.

Although the display device of this embodiment may be a liquid crystaldisplay device or an EL display device, an example of an electric paperusing an electrophoretic element is described here.

FIGS. 27A to 27C illustrate cross-sectional views in which an activematrix electronic paper is used as a display panel. The electronic paperhas visibility as high as paper, consumes lower power than other displaypanels, and can be made thin and lightweight.

FIG. 27A is a cross-sectional view of an electronic paper using a twistball method. The twist ball method is a method in which sphericalparticles each colored in black and white are arranged between electrodelayers included in a display element and the orientation of thespherical particles is controlled with the use of a voltage between theelectrode layers to display an image.

The electronic paper using the twist ball method which is illustrated inFIG. 27A includes a display portion 1114 and a driver circuit portion1116. The display portion 1114 includes a first electrode layer 1104connected to a transistor 1102 over a substrate 1100, a second electrodelayer 1108 provided over a substrate 1106, and spherical particles 1110arranged between the first electrode layer 1104 and the second electrodelayer 1108. The spherical particles 1110 each include a black region1110 a, a white region 1110 b, and a cavity 1110 c filled with a liquidaround the black region and the white region. A space around thespherical particles 1110 is filled with a filler 1112 such as an organicresin. The second electrode layer 1108 corresponds to a common electrode(a counter electrode) and is electrically connected to a commonpotential line. Note that the driver circuit portion 1116 includes atransistor formed through the same process as the transistor 1102 in thedisplay portion 1114.

FIG. 27B is a cross-sectional view of an electronic paper using theelectrophoretic element method. In FIG. 27B, microcapsules 1118 are usedinstead of the spherical particles 1110 in FIG. 27A. The microcapsules1118 each include a transparent liquid 1118 c, a negatively chargedblack particle 1118 a, and a positively charged white particle 1118 b.Each of the microcapsules 1118 has a diameter of approximately 10 μm to200 μm.

In the microcapsules 1118 provided between the first electrode layer1104 and the second electrode layer 1108, when an electric field isgenerated by the first electrode layer 1104 and the second electrodelayer 1108, the white particles 1118 b as first particles and the blackparticles 1118 a as second particles move in opposite directions, sothat white or black can be displayed. A display element using thisprinciple is an electrophoretic display element. The electrophoreticdisplay element has high reflectivity, and thus, an auxiliary light isunnecessary, power consumption is low, and high visibility is providedeven in a dim place. Moreover, an image which has been displayed oncecan be retained even when no power is supplied to the display portion.

Note that the first particles and the second particles each include apigment, and do not move when there is no potential gradient. The colorsof the first particles and the second particles are not limited to blackand white, and any color may be used for the first particles and thesecond particles as long as the colors of the first particles and thesecond particles are different from each other (the colors includeachromatic color).

A solution in which the aforementioned microcapsules are dispersed in asolvent is referred to as electronic ink. The electronic ink can beprinted on a surface of glass, plastic, cloth, paper, or the like.Furthermore, by using a color filter or particles that have a pigment,color display can also be achieved.

Note that the first particles and the second particles in themicrocapsules 1118 may be formed using a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed using a composite material of any ofthese.

FIG. 27C is a cross-sectional view of an electronic paper using a methodusing an electronic powder and grain. Positively charged black liquidpowder 1124A and negatively charged white liquid powder 1124B arecontained in a space 1122 surrounded by the first electrode layer 1104,the second electrode layer 1108, and a rib 1120. The space 1122 may befilled with the air, for example.

With a potential gradient generated by the first electrode layer 1104and the second electrode layer 1108, the black liquid powder 1124A andthe white liquid powder 1124B move in opposite directions, so that whiteor black can be displayed. As the liquid powders, color powders of red,yellow, and/or blue may be used.

In FIGS. 27A to 27C, a light-transmitting plastic substrate or the likecan be used as the substrate 1100. Note that, as the plastic substrate,a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used, forexample. Alternatively, a sheet in which aluminum foil is interposedbetween PVF films or polyester films can be used.

When a plastic substrate or the like is used for the substrate 1100, forexample, a separation layer is formed over a glass substrate, an elementis formed over the separation layer, an upper surface of the element isbonded to a different substrate, and the separation layer is removed, sothat the element may be transferred to the plastic substrate from thedifferent substrate. Note that, for example, tungsten oxide can be usedfor the separation layer. It is preferable to form the separation layerusing tungsten oxide because separation can be performed using water.Note that the different substrate described above may also be a plasticsubstrate.

With this embodiment, a transistor having high field-effect mobility canbe formed over a large substrate, and a driver circuit and a pixelcircuit can be formed over one substrate; therefore, a high value-addedelectronic paper which is, for example, capable of double-frame ratedriving can be provided.

In this embodiment, an example of an electronic paper formed using anelectrophoretic element or the like which is one embodiment of thepresent invention is described; however, one embodiment of the presentinvention is not limited to this, and an electronic paper of anothermode may be used. For example, an electronic paper in which a liquidcrystal element or an EL element is used for a display element may beused.

This embodiment can be freely combined with any of the other embodimentsand carried out.

Embodiment 12

In this embodiment, described are electronic devices each of which isone embodiment of the present invention including a display portion towhich the display device described in Embodiment 11 is applied.

As examples of the electronic devices to which the display device inEmbodiment 11 is applied in a display portion, the following can begiven: cameras such as video cameras and digital cameras, goggle typedisplays, navigation systems, audio replay devices (e.g., car audiosystems and audio systems), computers, game machines, portableinformation terminals (e.g., mobile computers, cellular phones, portablegame machines, and electronic book readers), image replay devices inwhich a recording medium is provided (specifically, devices that arecapable of replaying recording media such as digital versatile discs(DVDs) and equipped with a display device that can display an image),and the like.

The display illustrated in FIG. 28A includes a housing 1200, a supportbase 1201, and a display portion 1202, and has a function of displayinga variety of input information (e.g., still images, moving images, andtext images) on the display portion 1202. Note that the functionincluded in the display illustrated in FIG. 28A is not limited to this,and for example, the display can be provided with a speaker, and thedisplay may be a touch panel through which information can be not onlydisplayed but input.

In the television set illustrated in FIG. 28B, a display portion 1212 isincorporated in a housing 1211. The display portion 1212 can displayimages. In addition, the structure in which the rear side of the housingis supported by fixing to a wall 1210 is illustrated here.

The television set illustrated in FIG. 28B can be operated by anoperation switch of the housing 1211 or a remote controller 1215. Thechannel and volume can be controlled with operation keys 1214 of theremote controller 1215 and the images displayed in the display portion1212 can be controlled. Furthermore, the remote controller 1215 may beprovided with a display portion 1213 for displaying data output from theremote controller 1215.

Note that the television set illustrated in FIG. 28B is provided with areceiver, a modem, and the like. With the receiver, a general televisionbroadcast can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

The computer illustrated in FIG. 28C includes a main body 1220, ahousing 1221, a display portion 1222, a keyboard 1223, an externalconnection port 1224, and a pointing device 1225, and has a function ofdisplaying a variety of information (e.g., still images, moving images,and text images) on the display portion 1222. Note that the computerillustrated in FIG. 28C is not limited to this function, and forexample, may include a function of a touch panel which can inputinformation as well as displaying information.

By using the semiconductor device described in Embodiment 1 in a memoryportion of each of the electronic devices described in this embodiment,electronic devices with high reliability and low power consumption canbe obtained.

By using the display device described in Embodiment 11 in a displayportion of each of the electronic devices described in this embodiment,electronic devices with high reliability and low power consumption canbe obtained. Accordingly, the aperture ratio can be improved.

This embodiment can be freely combined with any of the other embodimentsand carried out.

This application is based on Japanese Patent Application serial no.2009-242871 filed with Japan Patent Office on Oct. 21, 2009, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first line; a second line; and amemory element, the memory element comprising: a capacitor; and atransistor, the transistor comprising: an oxide semiconductor layerwherein at least a channel formation region is formed in the oxidesemiconductor layer; and a gate electrode with a gate insulating filminterposed between the channel formation region and the gate electrode,wherein one of a source and a drain of the transistor is electricallyconnected to the first line, wherein the other of the source and thedrain of the transistor is electrically connected to one of terminals ofthe capacitor, wherein the other of the terminals of the capacitor iselectrically connected to the second line, wherein a carrierconcentration in the oxide semiconductor layer is less than or equal to5×10¹⁴ cm⁻³, and wherein a value of off-state current of the transistoris less than 1×10⁻¹³ A.